Before I post what characters that I want to see. I would like to ask you some questions about this:
Q1) What do you mean when you say masking or a masker?
Q2) Can post more then one character?
Q3) Can I post both female and male characters?
Q4) Can they be any character from any media like any series, movies, animes, games, cartoons, etc?

1. By masker/possessor I mean the person doing the masking/possessing.
2. Yes
3. Yes
4. Absolutely!

Okay, then here goes:
1. How a huge male Yautja like this or this, this possessing Sailor Moon's body like this except it ended with the Yautja's huge and masculine body is warping and stretching her body into the shape of his body with a huge tight bulge forming of his junk at the crotch with Sailor Moon's hello kitty panties that is stretch so thin that it can't even hide it.
2.A huge muscular Bowser like this wearing Princess Peach as his skinsuit.
3.A huge and really hung male Cyberdemon from the Doom series possessing Bayonetta's body by forcefully entering into her vagina.
4.Bastion the robot from the game Overwatch is wearing a young woman as his skinsuit.
5.How Nasse the Angel from Platinum End getting possess by some Dino man creature like this by entering through her butt.
6.A armorless and hugely hung male Blackwargreymon possessing a teenage Kari or Rika from Digimon or a young woman through her butt.
7.A Gengar possessing a naked male Yautja by entering through all of his orifices similar to this that resulted transforming into a Gengar Yautja(basically a dark purple Yautja with a Gengar's face and tongue and it's ghost abilities).
8. General Horse from the game Puppeteer possessing someone and wearing as her skinsuit.
9.How about a small group of little demons possess a woman by entering through all of her orifices while she is a sleep the idea was inspired by this.
10.A sexy version of a very endow and huge Transformer that look like this or this possess a body of a human woman a wear her to disguise herself as a human.
11.Shendu from the animated series Jackie Chan Adventures possess the adult future Jade and turning her body into his original body from the inside kinda like in this comic here.
12.A feminine male alien that look similar to this took over a body of human woman.
13.A huge muscular Orc use this magic to teleport himself inside a body of a female elf wearing her as his second skin as his is huge body is stretching the elf girl's out over his body with his huge junk hanging out of her vagina.
14.Mewtwo possessing Misty's body by using his psychic power to lift her over him and sliding her vagina down on his head then sliding her down the rest of his body after Mewtwo is completely in Misty's body he gain total control of the naked Misty.
15.How a alien cockroach that look like this possess a woman by forcing himself into her body by forcefully burrowing into her butt and when the alien is inside he take control of the woman as he is now wearing her like a skinsuit that is tightly stretch over his huge ripped body with the woman's rear end is riding up deep in the alien's butt-crack and a huge bulge that look like it is barely holding is form at her vagina with the alien's huge penis is sticking out a little bit out of the vagina dripping cum.
16.How about a fox mask yokai, a spirit-like being that take a form of a fox mask that possess it's wearers to become his bodies the idea was inspired by this and this. The mask yokai need a body for him to use so possess a young woman by forcing himself on to her face the woman try to remove the mask but it wouldn't budge but it did something to her she manage to pull it off but also something pulling off with it. After taking off the mask she soon finds that she can't see or speak it turns out that the mask took away her face and as the woman is tumbling around in shock the mask is floating near her as he is amuse by her sad attempt to escape him then the mask started to brainwash her into his body she try to fight it but it was useless the mask was erasing her memory and personality making her think that she is his body and as while that is happening her body is taking a form of a huge humanoid male fox erasing any trace of her old features including her vagina replacing it with a huge human-like penis and balls after the transformation is complete there stand a huge faceless fox-like being standing with it's huge penis out. With the yokai mask satisfied with the success of his body let the body put himself on covering the body's faceless face after thoroughly examining his body if you what I mean is very pleased with the results decided to celebrate by having fun with his body then after him decided to go off to get some people to transform into his bodies to add to his collection. Note: The yokai's real form is a fox mask and doesn't have a body of it own so it transform it's wearers into many bodies for him to use as his body. The yokai mask's body is a independent extension of the yokai mask it can act on it's own without having to be tell what to do no matter the distance but it's act on the will of the mask but when the mask is on the body it is being use as his physical body when the mask is not occupying a body it will only have one tail showing the body itself is weak but when the mask is using the body would have nine tails showing it is now strong the body act as a conduit for the yokai mask to boost it's already powerful powers when it is using a body. And it can have many bodies over his control but it can only use one body at a time and all of it's bodies will have a form of a faceless humanoid fox with a huge junk and have one tail.
17.How about a male Krogan from the game Mass Effect discover magic and how to use it the Krogan use it to turn a human woman into a skinsuit and wear her as a disguise.
18.How about a male Geth from the Mass Effect game have the ability to invades the body of his victims by entering into the victim's butthole and controlling them from the inside like a skinsuit.
19. Firebrand the Red Arremer from the Demon's Crest game series possessing the body of Princess Prin Prin and wearing her as a disguise for Sir Arthur.
20.Gargos from the recent Killer Instinct game possessing Orchid's body from Killer Instinct.
21. Megabyte with a huge penis and balls from the show Reboot hack into Dot and infected her code from inside her by entering through her vagina taking control of her body.
22. Papyrus the living skeleton without his outfit becomes Toriel's skeleton of her body. A naked Toriel with big firm bouncing boobs have suddenly lost control of her body and her bones feels like it's change and something have replaced Toriel's skeleton with Papyrus and Papyrus is trap inside Toriel's body he can see through Toriel's eyes but he can't speak(Toriel still have control of her mouth). Papyrus's tall shape have stretch Toriel's body taller. Papyrus is naively and unawaredly making Toriel feeling up her boobs.
Well, that all of who I want to see being a possesser but I really like to see ideas 1, 6, 7, 11, 13, 14, 15, 16, 19, 21 and 22 happen.

You've got tons of ideas!

I do quite love the hyper, muscular, and masculine theme :)

I like 3, 10, 12, 13, 18, 19, 20, 21(I especially like this and even Hexidecimal looks very suitable),

Also, have you considered making your own OC?

Some big muscular hunk with a huge cock and balls?

Here some more Ideas:
23.How about the god Anubis possess a young woman by entering her through her butt to wear her as a disguise to hide among the mortals.
24.how about a huge muscular manly Tanuki that look like this use his Tanuki balls magic to possess a human woman teleporting himself into her body and his huge body is stretching out the woman's.
25. Boo Cat from the Tarot comic series wind-up getting possess by little demon imps by entering through her boobs.
26. Angel Blade from the hentai anime series got possessed by the Devil.
27.(this Idea was inspired by this) A young woman wind-up naked in a sex base toon-like world where the inhabitants are huge muscular sexy toons with huge penis(kinda like your Discordian character, Rex and like him they appear to have rubber-like skin that have a title sheen to them and they can stretch toon-like but unlike Rex they looks toony) there the woman's present caught the attentions of the toons due to her being the only non-toon they finds her non-toon looks very exotic and unique. So the toons soon start to swarm her and starts entering her body through all of her orifices when many have gotten into her it start to transform her all of the toons' penis and balls have replace her vagina with their many penis and her lips start to full out and her body got huge and masculine and a little tie and bowler hat appear on her the toons that are possessing her having started erasing her identity and personality replacing it a toon identity. When the transformation is done there stand a huge monstrous toon that like a mix of a orc and a reptile creature that have many penis and balls that doesn't remember even it's used to be human woman the toon completely unaware of the event that created him the toon try remember what he was doing before but he don't remember he went off to this business without a care in the world.
28.A naked Leo from the game Red Earth possess a little girl by entering in her through her vagina tightly stretching and warping her young body around Leo's into a huge muscular and male shape of Leo's to use her body to fight monsters (this pic here kinda make me think of this idea).
29.Bolban from the Bomberman games with a huge muscular anthro form with a huge human-like penis and balls possess Princess Millian's body by entering through her butt and wear her as a disguise for Bomberman.
30.Natia from the Bomberman games with a huge muscular body possess Princess Millian's body by entering through her mouth she would stick one foot in her mouth then the other then she would slip her up over her body stretching Millian's body into her huge muscular physique.
31.A Xenomorph that look like this or this possess Amanda Ripley from the Alien: Isolation game a new special type of a Xenomorph have caught Amanda and rip off all if her clothes then the Xenomorph forces itself in her vagina taking over body when it's is over now the Xenomorph now in control of Amanda's naked body with glowing red eyes as it walk off into the dark.
32.How about Vivian the living shadow femboi from Paper Mario: The thousand-Year Door(in the Japan version Vivian is actually a femboi, a very feminine looking male) possess Princess Peach's body by hiding in her shadow underneath her and suddenly attack her to try to get in Peach's vagina when he does get in her he take complete control of her with the naked possessed Peach is standing in front of a full-body mirror admiring herself and in the place of her reflection showing only the naked femboi Vivian admiring himself(idea was inspired by this).
33.How about a young human woman with her body getting invaded by aliens(idea was inspired by this and this). A race of aliens was searching through space for a new planet to call home and discover the planet Earth but the planet's atmosphere is deadly to them making them unable to live on the planet but after see the planet's dominant inhabitants, the humans so they to inhabited the inhabitants so the aliens build a machine that will distributes their kind into the planet's inhabitants' body. A young woman was enjoying her nice jog till a beam hit her allowing some creatures to hop into her body and each possess a certain parts of her body when it is over the woman found that certain parts of her body have got fuller and have eyes then her vagina started talking about how their machine is a success after seeing her own vagina talking the woman started screaming but quickly the vagina alien order a other alien to keep her quite so suddenly her mouth seal it own self shut one of the aliens have possessed her lips and are able to control the frighten woman try open her mouth with her hands they too are possessed by aliens. After silencing the woman the aliens then took control of her feet and force her to walk as the aliens is making her awkwardly walk she is silently sobbing to her self due to not having any control of her body as she walks off to somewhere unknown. Weeks later reports all over Earth have been about women having theirs body parts being taking over by aliens and most of Earth's females have been taken over by the aliens and they're now starting to take over Earth using the females' body and there are rumors of the aliens trying to find a way to also possess the human male body too.
33.How about a bunch of Boos possessing Princess Peach's body like this and this.
34.How about two huge muscular and very endow male demons both possess the same human woman. The young woman is jogging until she was ambush by two huge muscular demons with a huge penis and balls that are the size of watermelons and the two demons want to possess the woman but start fighting over for who get to possess her among the fight the woman try to get away but two demons manage to catch her before she escape. Now she is within the grasp of the demons as they continue to fight over her and in the middle of the fight one of demons jam his fist in her butt in a attempt to possess her but after the other demon see what he is doing he too also put his fist in the woman's butt both entering in her butt as they continue to fight each other and they both manage to completely enter her body but their huge muscular bodies is stretching her body tearing off all of her clothes as they still fighting inside her body looking like two huge muscular guys are sharing a small costume that is too small their huge bodies. Eventually the demons decided to share the woman's body so one demon manage squeeze himself into one side of the woman's body putting both of his legs in one of her leg stretching it's into a muscular shape and putting both of his arms in one of her arm stretching it's into a muscular too. The same for the other demon too as he squeeze himself into the other half of the woman's body and finally they manage to squeeze both of theirs head into the woman's head filling it up and stretching it big when it is done now standing is the woman stretch in a muscular form with both of the demons' huge penis is hanging out of her vagina and is dripping cum. Now the demons try to move her using teamwork as one demon can only control one half and the other demon control the other once they mastered of making her walk they makes her walk off into the woods where the demons come from as the woman is still fully aware of what demons are doing but have no control but cry silently to herself as they walks her make body into the dark woods.
35.How about Midna from the Legend of Zelda: Twilight Princess in her true form here but instead of having a vagina she would have a huge penis and balls like this possessing Zelda's body. Midna in her true form is hiding underneath a naked Zelda in her shadow then she jumps out of her shadow and enter in her vagina possessing her after Midna got total control of her now stand Zelda naked with glowing eyes and Midna's penis hanging out of Zelda's vagina.
36.How a Midna in her form with a huge penis and balls possessing her imp form self and wearing herself. Minda in her imp form is in a forest then suddenly a portal open up and something step out of it when the portal closed Imp Midna open her eyes to see it is herself in her true but she have a huge penis and balls instead a vagina. While Imp Midna is still in shock her true form self walk over to her and grab her then she suddenly force herself into Imp Midna's vagina entering in her body when Imp Midna snap out of her shock she try to stop her true form self but it's is already too late she is half way in her. When the true form Midna finally completely possessed her imp self the true form Midna is wearing her imp self and her form is stretching Imp Midna's body in the shape of the true form Midna's body with her huge penis hanging out of her imp self's vagina. And it turns out that the true form Midna is from the alternative future and she travel back to past to aid Link into stopping Ganondorf from taking over the Twilight Realm without messing up the timeline(that's the reason why Midna possessed her imp self it's is so she won't mess up the timeline. And she will have to make sure that she do exactly everything that her imp self did when she was aiding Link in his quest while without causing any suspicions in her true form while wearing her stretch out imp self as a disguise meaning that she will have to pretend that she don't know Link or what will happen on his quest and that's include always being completely naked and riding Link's wolf form exactly like when she was in imp form thus making her huge penis flop around freely out in open all to avoid messing up the timeline). I add a backstory to this idea because I just like sexy paradox time travel stories where the hero travel back in time to aid herself or himself without causing a paradox but really the hero travelling in back time is what the cause the paradox(like the hero accidentally kill her or him past self and to avoid messing up the timeline the hero pass her or him as past self and living the past self life without changing anything but it only causes a new and totally different timeline to be created with the hero being completely unaware of the changes to the timeline) so it would be funny to see what kind of new timeline from true form Midna's actions will be created after she help Link defeat Ganondorf.
37.How about a huge beefy and hyper-endow anthro killer whale that look like this possessing a young woman on a beach. A young woman is on a beach sunbathing but unaware a huge beefy looking anthro whale with a gigantic uncut penis and balls emerged out of the ocean and started walking towards to the woman who is completely unaware of the anthro whale as his huge junk is swinging out in the open. When the woman finally realize that something is standing over her it was too late the anthro whale grab her and rip off her bikini and stick his huge arm in her vagina next his other arm then his head when the anthro whale have completely enter into the woman's body after her body shape shifted back to normal the whale have completely possessed her with the woman's eyes glowing and the whale gigantic penis is hanging out of the woman's vagina as the possessed woman walk back into the ocean.
38.How about a hunky insect creature that look like this possess a helpless young female cheerleader's body by entering through her vagina ending with result of the possessed cheerleader's body being muscular like the insect creature's with bug wings and the insect creature's huge penis sticking out her vagina forming a semi-bulge in the cheerleader's small panties(semi because the stretch panties can still see the huge penis).
Well, that is all of my possession ideas for now. And as for your question No, I don't really have official OC characters but I do have many concept ideas for OC characters that others people can use(like one of them is a time displace/time hopping prehistoric tribal-like a barbarian-like hunky and hyper-endow caveman that only wear a small loincloth and is a tech-savy mechanic that uses his gadgets and advanced science knowledge that he learns from the advanced future in battle or the one about a human man that gains powers of a Digimon through a experiment from a evil shadowy organization but wind-up escaping the evil organization escaping into the Digital World and due to the experiments he don't remember who or what he was before he got caught by the evil organization and did experiments on so he now believes that he is a mutated Digimon and eventually wind-up getting himself a human kid as his tamer or the one about a anthro wolf become a were-man, basically a reverse of the werewolf concept and even the one about time travelling Orcs that policing time itself are just a few of my many ideas) if you want to see them I can show you them if you want to see more details of them.

Lots of good stuff^^

By all means. I'd love to hear more details.

Here some more ideas:
39.How about a bottomless Amber the bunny like here from the Armello game and art is here getting possess by a spirit while exploring a dungeon hunting for treasure and tumble upon a chest she open it releasing a spirit that possessed her by entering her body through her vagina the now possessed Amber have glowing eyes.
40.How about River the wolf from Armello get possessed by the dark Lion King with his dark powers he possess the naked body of River by turning into dark shadowy mist and enter her vagina when the king have completely possessed River the Lion King admire his new young body as he rules his kingdom.
And I forgot to mention that the Reboot idea was kinda inspired this here. And okay, so do you want me to post my OCs concept ideas right here in this journal or in this journal here?

Not played that game but I do like the concepts^^

I think the OC journal is a good place to post it.

I don't know why, but the first character I imagined doing this was Char Aznable from Gundam.

"I came here to laugh AS you."

I guess I'm not entirely sure what qualities would make a character a good masker/possesser.

Kotaro Fuma, YES!! Could also Hatori Hanzo play the "straight-laced".

Kotaro is sexy cakes

I'd say Medaka Kurokami from Medaka Box or Nui, Ryuko, or Satsuki from Kill la Kill (Nui's sort of done it once, though).

I know you've already done a story on him, but I think Jiraiya from Naruto would be quite fitting.

Masking, possession and shapeshifting all already exist within the Naruto universe. Jiraiya is a pervert and a skilled ninja. And if you like you could also make him younger and more of a hunk (he either simply used to be a hunk or he transformed into one). On that matter, you could also have him use some kind of unique transformation jutsu which is more fun/interesting and doesn't just go *poof* and instantly transforms him.

He even possesses and controls some man's body at one point in the actual anime/manga. If I remember correctly, we only get to see him de-possess the man by coming out of the man's shadow. But I imagine you could explain that Jiraiya might have other sexier ways to possess people, including methods that resemble bodysuits. But he is very sneaky about this, constantly hiding and/or in disguise, so as not to get caught while infiltrating women's bathhouses. This is why we don't see that side of him in the manga/anime. He is just that good at being a master of disguise

He could have all kinds of hidden "pervy jutsus" which he just has never shown to anyone, because he hasn't met anyone who would appreciate them. Even if the story takes place at a point when he knows Naruto, you could argue that Jiraiya only intends to show Naruto this stuff when he considers him "a man", or something like that.

All very true.

*Poofs* are abominations

Honestly, I'd keep him at his older age for more contrast to his hosts/disguises.

Are you familar with Invader GUR's ghost work? I could see Jiraiya having maybe a ghost jutsu similar to that.