In the anime, Elysion was a land supported by the energy of the beautiful dreams of humans. Queen Nehellenia and her minions from the Dead Moon Circus invaded Elysion in search of the Golden Crystal, but because she did not have a beautiful dream, she was unable to take it. Instead, she captured Helios and locked him away. When he saw the light of Rini's beautiful dream, his spirit took the form of Pegasus and took refuge in it.
As Pegasus, he gave Sailor Moon the Kaleidomoon Scope and Sailor Mini Moon the Crystal Carillon; using the Carillon, Sailor Mini Moon summoned Pegasus, allowing Sailor Moon to use his power to perform Moon Gorgeous Meditation. He revealed his identity first to Rini, in episode 158, and then in episode 162 to the Sailor Scouts and Tuxedo Mask.
When the Dead Moon finally discovered that Pegasus was hiding in Rini's dream and captured her, Queen Nehellenia cast a spell on the girl to put her in an eternal sleep. In episode 165, Helios gave her a kiss that revived her. She then used the power of the Golden Crystal, with the help of the people of Earth, and destroyed the Dead Moon Circus.
Helios then left to return to his duties in Elysion, but gave Rini a kiss on the hand before he left, leaving the Inner Scouts to remark in dismay that even Rini had a boyfriend before they did.
In the manga, Helios lacked some of the power he possessed in the anime, as he did not carry the Golden Crystal and could not use its power. However, he was the guardian of the Earth and Elysion, and by extension, Darien Shields. As the Earth, Helios, and Darien's spirits were tied together, when Nehellenia invaded Elysion and imprisoned its guardian, Darien fell ill.
Helios was looking for his "maiden" (whom he thought was Sailor Moon) who could break the seal on the Golden Crystal. It turned out, however, that the maiden who had approached him was Princess Lady Serenity, the future form of Rini.