This sounds like a cool Idea, love the concept art ^_^
Hmm, I personally would love it as a companion style character :O!!
I think maybe he should be a boss, on DD he might be at a certain layer.
Survival he spawns after a certain amount of time.
Creative he is spawnable and you can customize the condition's for it.
Now when you first meet him, he attacks you like a hostile mob. If you have greenstone you can offer it to him. if not you must run away or fight back , Once he accepts your greenstone or reacjes a certain level of health ( he is not killable maybe?)
Anyway after that, he will stop his attack and dissapear.
After you build him a shrine, and offer your greenstone to him he will appear once again :O!!!
Then he speaks saying that he is glad that he met a worthy warrior if you beat him or
"Thankyou for the offering" if you gave him greenstone ^_^
(sorry am rambling
Maybe as your companion he can fight mobs for you, and just kinda be thier with him.
Befrending him could unlock say a blueprint or Item for you to use so you can summon him into battle?\
Would love to keep talking about this ^_^