Like in real life we have bros or girls that does everything together, without limits. In this case, which anime character would your bro or other side of the pair that you would like to literally anything together? Try not to say characters that utterly stand out because of their powers, actions, and ideals.
Example to avoid from: DUDE, I WOULD WANT LELOUCH TO BE MY BRO, THAT WAY WE CAN FUCK THIS WORLD UP MAN!!!
Another example for avoid: Man, I'd get Azusa(K-on!) to be my pair, that way we'd fuck anytime and still be cool with it.
Treat the character from their personality alone, not your personal gains/benefits. Note that it might be difficult to do so be cautious of who you would chose.
I'd think Haruna(Mahou Sensei Negima) would be my other missing part to make a pair. Same interests, same drawing skills, hell I would even bust out a complete nerd/geek mode and she'd totally understand me and probably get into opinionated wars that leads to nowhere. Her personality seems really fun and I think I'd never get bored of her girlish schemes that she creates for Nodoka, and probably join along on the fun. I'd probably do the same for her, just for her and maybe for laughs.