I don't headcanon much...but when I do, it's Jax/Fizz. (Who both belong to Riot Entertainment or whatever, they're characters from League of Legends, a popularish-I-guess kinda online MOBA, go play it, if you want.)

This was drawn more so as to get out what was in my head, don't take mer srsly...but if you like it, I love you too. x//3

Not just relationship-wise, but also in my headcanon, Jax is actually from the civilization that Fizz was from before it was destroyed by the sea monster.

All the reasons:
They both handle long weapons.
They both have jump abilities.
They both have dodge abilities.
They both have abilities clearly showing off their mastery of fluid movement.
They both have three pronged fingers and feet.
They both twirl their weapons.
They both egg on their opponents.
They're both adept fighters.
They both have abilities that empower the weapons they use.
They both have fun when they're fighting.
They both have names that end in 'sss'.
I want to fuck them both, and/or vice versa. All long weapon puns required.
The above reason is the best reason, and everything else is sidenotes that are important I guess.

...My headcanon is that long ago, Jax was part of the military and general defense of the civilization that they both lived in. And in this civilization...nobody EVER grew up to be Jax's size and form. If anything, the fact that he started to look like the humans that would throw their nets down to sometimes catch them and sell them as oddities had to have been a bad omen. The oracles of the civilization even foresee that a great tragedy involving too much power was coming soon, and everyone being highly suspicious of Jax and his disgusting form (to them at least), he was cast out.

And then later, washing up on shore, he'd meet Ashram, one of the people in charge of hiring people to the League of Legends. And through some event, most likely protecting Ashram, he'd show his combatitive skill (which was high even for their underwater fighting race), and Ashram would agree that he could join. He wears the clothes to hide who he is, even making them super bulky to hide his normally somewhat lanky build, and has Ashram spread the rumors about Jax being a soldier-for-hire and such to cover his tracks.

The people back in the civilization are living their lives normally, and then...hahaaaaa. Too much power meant that the sea monster was too powerful, and they just happened to kick out probably the one guy who could kick the monster's ass. Fizz comes home after playing outside, sees it's gone, takes up a trident...

...and the rest of it is history. =w=

...I veeeery rarely headcanon. But I love this one dearly. Oh, and of course, Fizz is of consenting age...Another fun thing I like to think about is if Fizz is actually older than Jax, hehehe. Poor blushy Jaaaax~