SECOND BEST FRIEND! To be honest if it weren't for the fact Ren is Seto's friend and Seto had been looking for her for so long, Crow might have treated Ren pretty badly when they met. ...which he did until he realized she was Ren! But while it's true he became Ren's friend because she's Seto's important friend, he really does like her. And honestly? He likes playing big brother to both of them and telling them all about the things they don't know, even if he's completely bullshitting about them. Seto was the first person he ever met, Ren is the second, and he really feels like the three of them are family (or at least something close to it, since it's not like he knows what family is exactly like).
Ren is nice and cute, and he feels special since she doesn't jiiiii at him and gives him he treasures :D!! I think it's a bit hard to really describe his feelings for her, but to sum it up he genuinely loves her like a sister, pretty much. And he would really, really do anything to protect her, just like with Seto! There's also the fact Crow doesn't know how long he'll be able to stay with them, he knows he shut down once, and he knows it could happen anytime! He doesn't feel his batteries low at the moment, but now he does realize this is some Camp freaky thing and that, probably? If he were to get out of Camp he'll be dead again. And that makes Ren the person he counts on to take care of Seto when he won't be there anymore.

TL;DR Ren is also a precious friend and he'd shank people for her.