Mutsumi Hoshikawa served in the Earth Defense Army's medical unit before joining the Science Guard Party. Her fascination with unexplained phenomena drove her data gathering, securing her team position. She maintained a prior connection with Hiroaki Tobe.

Mutsumi possessed a gentle, resilient, and cheerful personality. She adamantly refused special treatment due to her gender, advocating for equal standing and taking pride in her role. During combat, she demonstrated skills on par with others, piloting vehicles and investigating threats. Outside missions, she showed warmth toward animals and children, though she struggled with tasks like brewing coffee.

Her history includes critical missions. Investigating an Antarctic iceberg, she witnessed Ultraman Joneus' debut, expressing early belief in his benevolence. On a joint tornado investigation, she was swept into a vortex, using a pendant gifted by Kei Marume to signal Joneus, aiding in defeating Spiral. Facing the ancient bird King Moa, Mutsumi pleaded for its life after it mistook her for its mother; Joneus shrank it back to its original size.

Mutsumi gradually developed feelings for Choichiro Hikari, often collaborating with him. She noticed inconsistencies in Hikari's behavior and his connection to Joneus, particularly after a mission where Hikari left her side during a crisis, resulting in her near-fatal injuries. Her heart stopped, but Captain Daisuke Gondo revived her. Hospitalized, she identified an enemy command ship controlling Gedon and relayed this intelligence through a communication device, enabling its destruction.

She participated in pivotal battles against the Heller Empire, including the final assault on Planet U40. After the conflict, she returned to Earth and bid farewell to Joneus and Amia. Her abilities included medical training, traditional painting, and proficiency in operating all team vehicles.