If Link tries to talk to Fifi while Link is in his regular form, Fifi will just bark at him. When Link is in Minish form, he can talk to Fifi and Fifi states that Stockwell has been coming home very late and is not able to play with her, causing Fifi to become lonely.
Link is able to fuse Green Kinstones at various points in the game with Fifi. As it is a Green Kinstone fusion, it is not set to a specific treasure chest, but instead, one of the many possible treasure chests. There is one random Red Kinstone within the game that opens up a location in the Hyrule Castle Garden leading to a Piece of Heart, as well as a random Blue Kinstone that opens up a tree in North Hyrule Field leading to a Fairy Fountain. Fifi is one of the many characters in which Link can make this Kinstone fusion with.