@Hello _its_me_again White hair guy with fancy hat is trying to raise the three children into good people (black hair was originally a villain, I don't remember what role the female brown hair and male brown hair have, but they're not protagonists). He's offered two of them (brown haired siblings), with their consent, to be someone else's disiple so they can extend their knowledge further, and black hair wants to stay by his side. The brown hair we currently see is the sister.
Someone correct me if I was wrong please, thank you!.