Maybe he won rock-paper-scissors? [Jataro's thinking like you are! At least a little.] Maybe you should ask Big Sis Peko and see what she says. I don't have any idea why he leads instead of her. Or instead of anyone else. Big Sis Sonia is a princess...Big Bro Fuyuhiko is a yakuza...wouldn't they be good leaders?

[Meanwhile Hajime is...what? Nothing, right??]

If he's not human anymore, maybe he really should be a servant. That's what robots are. But...I don't think I'd want him leading any kind of rebellion against humans...so maybe not.