Rikka hails from the 1098th Battalion of the Galactic Revolutionary Alliance, crash-landing on Earth after a battle with the Interstellar Empire. To conceal her origins, she adopted the identity of a middle school student at Kashihara Girls' Academy. A memory-manipulation device hidden within a stuffed animal fabricated her enrollment records and friendships. Her true form features beige hair, red eyes, a snaggle tooth, and pink ribbon-like respiratory tendrils on her head, vital for her chlorine-based biology. This physiology produces bright pink blood and toxic, chlorinated-gas flatulence.

Her alien nature grants rapid regeneration from fatal wounds, immunity to toxins, psychic abilities enabling mind control via the stuffed animal, localized reality manipulation, and limited time travel. She wields a purple water pistol that liquefies targets but fails against humans. Social integration is complicated by her literal interpretation of human behavior, limited understanding of Earth customs, and accidental power activations—transforming excluded classmate Sekine into space hogs or altering the school's gender balance after misreading Tsubasa's crush. Sekine often reversed these mishaps.

Anxiety over maintaining her disguise causes physical signs like drooping tentacles. Her commitment to secrecy wavered, leading her to interrogate friends with the device suspecting an impostor, or nearly facing exposure when enhanced intelligence was granted to Chiyo. During a climactic confrontation, Rikka initiated departure to protect her friends but halted her escape upon recognizing their mutual acceptance transcended her deception. This reconciliation cemented her bond with the group, solidifying her choice to prioritize Earth relationships over returning to space.