E. Aster Bunnymund
This is a complex relationship. We don't know what kind of relationship there was before of the blizzard of '68, but after that Bunny just keeps holding a grudge against Jack, and when he's chosen as the new guardian by the Man in the Moon, he wishes for the groundhog.
Bunnymund has this hate toward the cold and snow, which makes it difficult to like Jack, what's even more difficult is that Frost likes to irritate Bunny by teasing him and making fun of him, which doesn't help his cause and makes Bunny say things like throwing in Jack's face that people believe in Easter Bunny, but not Jack, because that's the only way he knows how to get back to the boy.
The things slowly begin changing, after Jack helps the guardians with children, because while they protect them, they forgot how to actually be with them, and the Bunnymund is the one that gets involved the most, taking quite a likening to Sophie, Jamie's sister. He, after centuries, has fun again.
But their relationship changing in friendship and defines both is, when Jack makes Jamie believe in Easter Bunny, when the boy almost lost completely his believe, because of Pitch's scheme. And then, when Bunnymund thinks Jack will make fun of him, Jamie tells him that it was Jack who made it possible for the boy to keep believing.
And in the end, when finally Jack is to take the oath and become officially
the Guardian of Fun, we can here in the back, how Bunny says: that's my boy.