He's a Koopa turtle who can control the clouds. He wears a big pair of glasses and his shell has stripes on it instead of the Hexagonal pattern of most other Koopas.
In Super Mario Kart, Lakitu was a useful character for Nintendo because he could fly around on the screen without getting in the way and rescue you too, thanks to his cloud. I expect it could have just of easily been a Koopa Paratroopa or something, but Lakitu looks friendly enough. In following games, Lakitu then started to take on more and more roles as a friendly and helpful character, not always fighting. But don't forget, there were mean fighting Lakitus in SM64 as well.
