Choichiro Hikari served as an astronaut aboard the space station EGG3 before joining the Earth Defense Army's Science Guard Party. During a mission, a meteor shower struck the station, damaging its outer wall and breaching his spacesuit. Suffering critical oxygen loss, Hikari prioritized his crew's safety and repelled a Goglan larva threatening the station. Ultraman Joneus witnessed this act of selfless bravery and chose Hikari as his human host, impressed by his courage and humility in keeping the incident secret.

While traveling to Earth to join the Science Guard Party, Hikari encountered a mysterious light. He merged with Joneus upon learning of an impending crisis threatening Earth. This merger allowed Joneus to operate on Earth while sharing Hikari's body. Hikari transformed into the Ultra using the Beam Flasher device on his forehead. He initially believed the encounter was a dream until his first transformation during a mission against the monster Seagra confirmed the reality of their partnership.

As Joneus' host, Hikari maintained rigorous daily physical training to support their merged form in battle. He struggled with the secrecy of his dual identity, especially when teammate Marume noted his frequent disappearances during monster attacks and criticized his absence. This tension culminated during a battle against Goglan. Hikari hesitated to transform, hoping to fight using his own strength. His indecision resulted in Marume's injury and city destruction, causing Hikari intense guilt. After crashing his jet pursuing the evolved Goglan, Hikari faced near-fatal injuries but transformed after Joneus expressed faith in him, defeating the monster.

Hikari's resolve faced another test confronting the powerful monster Gerada. Ignoring Joneus' warnings about its strength, he transformed and won but collapsed from exhaustion. This left the team vulnerable when Gerada's remains formed a new monster, Janyur, highlighting the merger's physical toll. Later, during a battle against Bedran, Hikari and Joneus were killed after their energy depleted completely. Elek, another Ultra from Planet U40, retrieved their bodies and transported them to U40 for revival.

Revived on U40, Hikari learned Joneus' true identity as the strongest warrior of U40 and the planet's connection to Earth's history. He participated in defending U40 against the Bader Group's invasion, collaborating with Amia (Joneus' sister) to pilot a fighter craft. During this conflict, he heard Joneus' revived voice and witnessed his recovery. Hikari trained in U40's "Experience Dome," studying its culture and deducing a link between Earth's Spirit Parasites and the Bader Group. His memories of U40 were initially erased upon returning to Earth to protect the planet's secrecy, though subsequent events involving Amia's presence and the Heller Empire's invasion restored his awareness.

Hikari developed romantic feelings for Amia during their interactions, which she reciprocated. As the Heller Empire escalated attacks on Earth and U40, Hikari's teammates gradually deduced his identity as Joneus' host due to his recurring absences during crises. In the final conflict against the Heller Army, Hikari and Joneus separated to allow Hikari to join the Science Guard Party's raid on Heller City. During the mission, they remerged to survive a fall from a tower and ultimately defeated the invaders.

His character evolved from a dedicated but conflicted host to a confident leader who reconciled his human responsibilities with his role as an Ultra's partner. Throughout his journey, he maintained traits of bravery, compassion, and a strong sense of duty, consistently prioritizing others' safety above his own.