My quick theory:

That super powerful alien (that Mori has the mark of on his cape) or one of its close underlings fell in love and somehow had a child with a human. Something happened to the human partner so after they found Mori in that basement they decided a human orphanage would be better for him to grow up in since he's more human-like than alien. But when he disappeared they obvi went looking for him. I think the one we saw recently wasn't the parent though, just works for the parent.

I have a bad feeling the human parent might be dead. If that's the case, I hope Rider becomes Mori's other adoptive parent.