Kaja Bergmann is a ghost who perished in the May 29, 1945, bombing raid on Yokohama during World War II. A German exchange student at the same Japanese school as the primary ghost Arashi, she became Arashi's best friend. Kaja retains the appearance of a sixteen-year-old girl.

She shares Arashi's time-traveling ability, which necessitates forming a "connection" with a living person from the present. Kaja's inherent shyness around boys restricts her connections solely to living girls, defining her potential partnerships. Her personality presents a polite and cool demeanor, contrasting with Arashi's lively carefree nature, though deeper emotions lie beneath this composure. Kaja harbored feelings for the owner of the 1945 café "The Ark." Attempting to save him from the bombing, she traveled back in time with her present-era partner, Jun Kamigamo. Tragically, their intervention accidentally caused him to leave the café's safety, leading to his death in the very raid she sought to prevent.

Kaja forms a significant bond with Jun Kamigamo, her designated time-travel partner. Jun, a girl who initially presents as male to others, is among the few aware of Kaja's ghostly nature and connection requirements. Their partnership enables numerous journeys to the past. Kaja occasionally exhibits an "Onee-sama" aura towards Jun, suggesting a protective or mentoring aspect to their dynamic.

A consistent character detail is her fondness for ice cream, noted as her favorite dessert. Her ghostly existence, like others in her group, is tied to the summer season. Her narrative involves confronting the trauma of her death and the loss of her loved one during the war, adding emotional weight to her time-travel missions and present interactions.