The question I'm really curious about is this: How much of Cain's development over the course of the miniseries is a matter of Cain changing, and how much of it is simply him accepting the rest of the group as equals and allowing them to see the inner parts of him that have been there all along?
One thing at a time for me here.
I think that we're seeing how Cain was prior to being canned, so to speak. I bet that those years in the iron suit just ate at him and he re-visited every opinion he ever made, every idea he had. Once he was free, and he began to accept them, he slowly turned back to the personality he had prior to Zero locking him up.
DG, Glitch and Raw, and possibly Tutor, became what the Tin Men were to him.
I like the armor comment because it works so well with him. He's obviously got a tender side because he's trying to reach out to them, he says things that make Glitch smile, he tries to comfort DG, he's willing to cry in front of people.
I agree he's the heart of the group, without a doubt. That especially shows after DG begins to lead - which I think is in the Resistance Members' Cabin. Once she begins to take that role, he falls out of the guide/leader position and more into his part as the heart of the group.
I had thought that Glitch might be DG's mentor, and in some ways he is, but Cain is as well. DG didn't grow up in the O.Z. - it's not hers yet, but seeing Cain being hurt makes it more her own. She learns how to care (heart) from seeing Cain hurt.