This is Don, the stupid cousin ghost!
Don
Donny Don Don

No, but seriously.
He's Rigby's cousin that died in an accident, trying to impress a girl.
That's right; he died being a simp. 😑

He shows up later after an important plot point for our main character occurs.
Don typically shows up to annoy Rigby by being an ass. Ironic.

Other times, he'll "babysit" him when he's in the Ghost Zone (read: drag him along on misadventures that usually get them into trouble. Usually for a girl. Moron. 🤦)

Outside of that, he has no ill will towards his younger cousin, he's just reckless. (Re-read how he died again.)

The rest of the family is ashamed of him and tends to pretend he doesn't exist/show annoyance when he's around. For good reason, but kinda harsh. Dude's a ghost, guys. 😬
(He probably only hangs around the kid at times because he's the only friend he has. Yikes.)

If you're wondering why his outfit and design are simple let me answer that. No ghosts look like regular people. It would have been funny to see a ghost permanently stuck in a wife beater for all of eternity. Or an embarrassing meme shirt that's no longer relevant. Forever. 😈
I know the thing about ghosts being able to change their appearance, but like...

Being stuck in the clothes you died in??? That's kinda yikes, bro. 😆 *hurries and changes into a better shirt* Now, I'll keep this on until I die! :)

(About his name. It was originally going to be something like Raphael or Ronaldo, but uh... Yuck. My friend off-handedly joked how it show be Don (like Rigby's brother on Regular Show) and I said, "You're on!" It stuck. So, yes. It's a Regular Show reference.)
(Eeugh, I hate that Don... 🤢)

Edit: His left arm looks so short now, I effed up badly. 🤦🤦🤦🤦🤦

*Googles "can you fuck a ghost"*😛
*forgets that this is Rigby's cousin* 😬
*Rigby is me* 😶
*Sweet Home Alabama plays* 💀