depends on where you're coming from...leo has a dark heart in all actuality, he just doesn't lash out at people like raph does... leo only appears to have the least amount of personality because he's forced to assume the leadership role thus making the burdon of responsibility lie on his shoulders... he can't afford to be the goofball or the hothead of the group because he's gotta worry about the rest of his bros...
leo has an honor code and training ethic that I find very similar to my own...that's why i like him.