Barbatos was confused when Diavolo ordered him to serve and protect you and only you...Diavolo essentially told him to completely focus on you, and he had no idea why...When he met you though...He figured it out...

You're a rare hybrid, a mix between a demon and an angel, probably the only one of your kind, Diavolo must've gone to convince your family to transfer you to Devildom...He's nice to you for the most part but he doesn't necessarily care about you...Well, maybe he cares a little...