Kaja Bergmann is a ghost who perished alongside her close friend Arashi during World War II air raids. A German exchange student in Japan at that time, she retains the spectral appearance of a teenage girl decades later. Kaja possesses the power to travel through time, but activating this ability requires forming a "connection" exclusively with living girls due to her inherent shyness around boys.

Her consistently polite, reserved, and cool demeanor contrasts sharply with Arashi's lively and carefree nature. Kaja deeply values her friendship with Arashi, forged through their shared wartime experiences and ghostly existence. She harbors a fondness for ice cream.

Kaja forms her necessary connection with Jun Kamigamo, a girl initially presenting as male to others. Kaja is among the few aware of Jun's true gender early on. This bond enables them to journey through time together, undertaking missions primarily focused on rescuing individuals from historical tragedies, especially WWII air raids. Their partnership develops a supportive dynamic, with Kaja occasionally exhibiting an "Onee-sama" presence toward Jun.

They utilize time travel not only for rescue missions but also for mundane activities like accelerating tasks or solving daily problems at their workplace cafe, a practice Jun sometimes questions. Kaja's background as a foreign exchange student occasionally influences her adjustment to the present day and interactions more noticeably than other ghosts like Arashi.

As Jun grapples with her identity and developing feelings for Hajime Yasaka, Kaja provides stability and understanding. She remains a consistent figure in Jun's support network, including during an incident where Jun temporarily undergoes an overnight age-up, appearing as an older, more feminine version of herself.