Luigi
For many years Luigi stood in Mario's shadow. These days he's made a name for himself and made his way into people's hearts. He drives, he palys mini games, he vacuums up ghosts and he helps his brother beat up bean people. He's pretty busy nowadays.
Sonic
Sonic the hedgehog put the sega genesis on the map. He has starred in many successful 2d platform games adn a few unsuccsessful 3d platformers. People still buy all of his new games. Developers know what people like. And what people like is Sonic.