Tenjouin Katsura, adopted and raised from infancy by Kaoru Yamamoto, originates as an infant escape pod survivor from the destroyed Planet Noah. Daigo Hoshiwatari discovered her and passed her to Yamamoto. Her alien Noahian physiology endows her with exceptional abilities: superhuman strength enabling her to lift 100-ton objects single-handedly, instinctive martial arts defense triggered by danger, clairvoyant visions during dimensional travel, and telepathy. She remains unaware of her extraterrestrial origins for much of her life.

Physically, she is distinguished by bright blue hair often in a ponytail and purple eyes, with a character design incorporating influences from Katsura Mafune and Sailor Moon aesthetics. Katsura exhibits a cheerful, affectionate personality that positively influences others, though she displays obliviousness to romantic advances, particularly Goro Hoshiwatari's longstanding feelings. Despite her kindness, her poor culinary skills are deemed hazardous by the crew, who prevent her from cooking.

Her narrative arc involves Yamamoto withholding her true origins while anticipating her eventual return to her people. After safe interplanetary travel is established, she departs for Noah but continues aiding the YAT crew during crises. This implies an acknowledgment of Goro's confessed feelings by the series conclusion. Recurring elderly passengers Mr. and Mrs. Toukichi frequently appear during her tenure as YAT's flight attendant and mechanic.