Nicholas St. North
He is the Santa Claus, and Jack Frost had tried to enter his workshop many times, but the Yeties always stopped him, and North didn't know it.
From the moment MiM shows them who the new Guardian is, this man accepts it, and begins with working on getting Frost as soon as possible and making him join. Which doesn't go as he plans, Jack doesn't like the music and doesn't think he is Guardian material, but North is the best in what he does, so he gets Jack to follow him, and tries to explain the Guardian's job. He is the one that makes Jack think and actually in the end to come to an answer.
He genuinely likes Jack, doesn't really understand when Frost is sarcastic, and treats him sometimes like a child sometimes like an adult.
The fact is that I really don't know how to define the relationship, because North is a madman, and so is Jack, so I think they actually are great friends because those two are two madmen, and wonder and fun go hand in hand together.
In some way we can also see that there is great respect from both parts, but
especially Jack's after he sees North's ways (and I'm talking just combat),
and how the jolly man sees the world, and accepts it, seeing wonder in everything.
He makes Jack become who he is in the end of the movie, accepting himself and
finding his center.