KK hates Ren because he thinks that she is a filthy misogynist and KK is a feminist.
He also finds Baxter stuck up and annoying, and thinks he's an idiot.
He does tolerate them, however. Initially he declines the invitation to join the club, though ends up joining despite any animosity because he had a lot of fun playing with them in walks and he is slowly warming up to Qiu. He also just gets a slightly giddy feeling from the thought of being in a club for BOYS in any capacity (unknown gender euphoria).