Man in the Moon
And again there is not much to say, the relationship between the Man in the Moon and Jack is strange one. While MiM does keep an eye on Jack, from the moment that he drowns in that lake, bringing him back to life, the only thing he tells him it's that his name is Jack Frost.
For the 300 years Jack keeps talking to the Moon, but he never ever gets an answer from it, it's like MiM wants him to find his way, without help, but then Pitch happens, and he has to tell the Guardians that he has chosen a new one and it's Jack Frost.
In my opinion, Mim had the idea of Jack becoming a Guardian from the beginning, but he had to test the boy, because he couldn't just made him Guardian the moment he was born in the spirit world, aside, Jack had to learn how to use his powers before he could do anything else, and he should have known what solitude and nobody believing in him was, so he would cherish even more when he would get it.
MiM gives me the idea of the parent whose idea of teaching a child to swim
is to throw them in the water and say: now survive.