In the Show? Gaz. They Fixed her up in the Comics, but she was just kinda of a bitch in the show. Inwas never really a huge fan of the "Wise-beyond-her-years-smart little girl" trope and Her relationship with Her brother felt a lot more mean spirited. It wasn't "Idiot Brother who greatly annoys her and needs a smack upside the head to set him straight but she cares for because he's family" like in the comics but "Stupid Asshole who I would Gladly get rid of myself if it wasn't for the fact that it make my dad sad and require me to stop playing games". That and, like Gir, she got to avoid most of the Slapstick and punishment, which is a neccesary part of making an Asshole character endearing ex. Pic related.
Although, now thanks to the Comics, Gaz is one of my favorites.