My character in Mario Kart depends on the game. If it's Super Circuit, I play as Peach. If it's DS, I play as Yoshi. If it's 7, I play as Metal Mario for time trials, and Koopa for VS races.
The only game I ever tried to speedrun was Pokemon Black. I think my play time was somewhere around 9 hours when I beat the game, so it wasn't bad, but it wasn't good.