Sonic, Knuckles, Shadow and Silver even Scourge the Hedgehog from the sonic series both game and comic.
Danny Phantom and Dark Danny
Tai and Matt from Digimon and also Dark WarGreymon
Sesshoumaru and Inuyasha
Trunks DBZ XD
Bulma from Dragon Ball through the entire series, until GT then it was Pan. Bulma because she was cute and intelligent (then quite HOT and intelligent), Pan because she was strong, determined, stubborn and had a big heart. I didn't like Bulma's daughter though because she was depicted as the stereotypical 'dumb blonde teenager' type always obsessed with clothes and whatnot...