She is the connection back to children for the Guardians, but especially for Bunnymund. The first moment is awkward, because he doesn't know how to interact with her, nor do the others. But after one of the fun snowflakes, that Frost uses on Bunnymund, he takes Sophie and shows her his world, and egg painting.
Sophie is quote taken by Aster, and whenever she sees him she is all "Bunny, hop hop". She is excited and follows the buck, while he shows her everything.
Bunnymund in the end is quite taken by her, he looks at her in awe, and probably
also thinks, how he could forget about this feeling, and the fact that they
do fight for the children, they forgot how to actually be with children and
treat children right.