Kaeru is an original character created specifically for the anime movie adaptation, appearing exclusively in narrative events occurring after the original manga's conclusion. His role centers on interactions with Nezumi and Mogura during the period following the Crocodile's death, forming new relationships as they navigate daily life after experiencing loss.

Functioning as a narrative device, Kaeru explores themes of continuity and moving forward. Symbolically described as a "fresh wind" entering the established world, he introduces new dynamics to existing character relationships. His presence facilitates depicting how Nezumi and Mogura adapt to changed circumstances, highlighting their ongoing journeys through grief.

Kaeru's specific background, physical characteristics, and origins remain undefined. His narrative contribution focuses on interactions and the perspective he brings to surviving characters, representing life's continuation and new connections after significant loss. He embodies the film's thematic exploration of resilience and the enduring value of daily existence.