Helios is the high priest of Elysion, a sacred dream realm beneath the Earth connected to Prince Endymion, and in the anime Sailor Moon SuperS he appears throughout the story in the form of a winged white unicorn known as Pegasus. His original duty was to act as guardian of the Golden Crystal of Dreams, a powerful treasure associated with the hopes and dreams of humanity. When Queen Nehellenia and her Dead Moon Circus invaded Elysion, Helios was captured. To escape being discovered, he willingly left his physical body behind and transformed into Pegasus, fleeing into the world of dreams and searching for a pure dream in which to hide. He found that refuge inside the dreams of Chibiusa, and from then on the two shared a deep and secret connection.

As a character, Helios is gentle, soft-spoken, and protective, with a quiet sense of longing for his lost home and his true form. He takes his guardianship seriously and does not reveal himself freely, asking Chibiusa to keep their meetings a secret at first. His main motivation is to protect the Golden Crystal from falling into the hands of the Dead Moon Circus, to shield dreamers from having their dreams devoured, and ultimately to free himself and Elysion from Queen Nehellenia's influence. His growing care for Chibiusa also becomes an important motivation, as he comes to value her safety and happiness alongside his larger mission.

Within the story, Pegasus acts as the secret heart of the season's conflict. The Dead Moon Circus attacks people who hold beautiful dreams in the hope of locating and capturing him, which drives many of the battles of the series. Whenever the Sailor Guardians face serious danger, Chibiusa can call on Pegasus to appear. He empowers Usagi and Chibiusa with their Super forms, granting Sailor Moon the Crisis Moon Compact and the Kaleido Moon Scope, and Sailor Chibi Moon the Chibi Moon Compact and the Crystal Carillon. He also lends his strength to Super Sailor Moon in combat against the Lemures. Near the end of the story, when the Dead Moon discovers that Pegasus has been hiding in Chibiusa's dreams, Chibiusa is captured and Queen Nehellenia places her in an eternal sleep. Helios reaches her and revives her with a kiss, after which Sailor Chibi Moon uses the power of the Golden Crystal, supported by the dreams and wishes of the people of Earth, to destroy the Dead Moon Circus. With the enemy defeated, Helios is freed from his Pegasus form and returns to Elysion.

His most significant relationship is with Chibiusa. He begins as a mysterious dream companion who keeps secrets from her, but over time the two grow close, and their bond deepens into something warmer than friendship, expressed through moments of trust, affection, and a final farewell. He also has a connection to Mamoru, since Elysion is the sacred land of Prince Endymion and the Golden Crystal is tied to that lineage. His abilities include moving through the world of dreams, flying as a winged horse, radiating protective light, empowering other warriors, and drawing on the Golden Crystal kept in his horn, which grants immense power drawn from dreams.