At least it's explained why he got his power: he's not the only one who fell and got saved by her, but he's the first one who didn't try to take advantage from Yggdrasil and even thanked her for the save. It's chlildlike mentality and naive, but seeing how Yggdrasil is sheltered and has basically no one to communicate, except her "mom", I can see how she would be easily captivated by that.
I mean think about it: you're stuck alone in a place for years, centuries and then after helping and meeting some jerks here and there, you finally find a decent person to talk, I'm sure anyone would be eager to at least bond with this person for a while.