Bunny and Sophie only ever first met the night before Easter Sunday. With Sandy supposedly dead after their previous scuffle with Pitch, the Guardians held all of their hopes on Easter to maintain faith in children. What they did not know was that earlier, Sophie snuck into Jamie's room while North, Tooth and Bunny where unconscious through Sand's dreamsand, Sophie found one of North's magic snowglobes. Seeing Bunny, she exclaims his presence, causing the snowglobe to send her to the Warren.
At first flustered and nervous about handling a child head-on, with a little help from Jack, Bunny was not only able to connect with Sophie, but was able to paint all of the eggs on time and remember why he was a Guardians in the first place.
She is later seen with the other children in their victory over Pitch, enjoying their snowball fight and saying goodbye to Bunny. During the credits of the film, the now-asleep Sophie was seen carried back into her room by Bunny's eggs that where left for her.
Much like Jamie is to Jack, Sophie holds a particular bond with Bunny. Not only is she one of the first believers Bunny has interacted with for years, but can also be said to be his favorite, and he her favorite of the Guardians. Whatever relationship they have from then on differs from story to story, the most of common being that he would visit her frquently, and when she comes of age, their relationship grows into "something more".
Note: Please add only ones that are part of the RotBTD fandom