Karen Tōno is the deceased younger sister of Kaoru Tōno. Her life ended in an accident after she fell from a tree while trying to catch a rhinoceros beetle. This attempt occurred as she sought to reconcile with Kaoru following an argument. Her death profoundly impacts Kaoru, burdening him with deep guilt and unresolved emotions stemming from his perceived failure to protect her.

Existing primarily as a memory, Karen serves as Kaoru's emotional anchor throughout the narrative. She embodies the innocence and happiness of Kaoru's life before grief. Their pre-death relationship is depicted as a close, typical sibling bond, marked by Karen's admiration for her older brother despite occasional disagreements.

Driven by rumors that the Urashima Tunnel could reverse death, Kaoru's desire to undo Karen's loss fuels his initial exploration. Within the tunnel, during a pivotal moment, Karen manifests to Kaoru in a fabricated reality. There, she reassures him that her happiness rests on him finding fulfillment with someone he loves, implicitly acknowledging his bond with Anzu Hanashiro. This encounter becomes the catalyst for Kaoru's acceptance of her death and his decision to embrace the future instead of clinging to the past.

Karen possesses brown hair, often styled in a ponytail and accessorized with a cap, along with blue eyes. She is consistently portrayed as a child wearing shorts.

Thematically, her character arc concludes through Kaoru's emotional resolution as he learns to cherish her memory without letting it consume his life. Her enduring influence persists as the catalyst for his journey toward healing and self-discovery.