Kerobby is a frog-like companion to Robby, prince of the Nave Kingdom. He begins with vocal reluctance about being sent to Robby's Land, questioning why he must stay in what he perceives as a provincial town against his will, his dissatisfaction evident through protest.

Kerobby possesses a unique ability: he can combine with various objects when placed into the knob on Robby's head. This merging process, called "合体練成" (combination refinement), forms new composite bodies specifically suited to overcoming the distinct challenges they face, central to their collaborative efforts.

Throughout their shared experiences, Kerobby and Robby consistently encounter troublesome situations. Despite their initial mismatched attitudes and frequent troubles, Kerobby gradually develops a cooperative partnership with Robby. Their joint efforts rely on "友情パワー" (friendship power), indicating their bond strengthens over time and becomes key to resolving difficulties, illustrating Kerobby's progression from early resistance toward active collaboration. Background details remain limited to his association with Robby, the Nave Kingdom, and his role within their shared adventures.