Guardian Priest Helios
Mamoru, the Earth, Helios, and Elysion are all linked to each other. When one falters, so do the others. Pegasus came to Usagi, Chibiusa, and Mamoru during an eclipse. He implored of their help and said that he was looking for the chosen maiden. Later on they found out that he needed the power of the Golden Crystal to save his world, Elysion, the world of Earthen dreams.
When the Dead Moon Circus begins to wreak havoc in Tokyo, Mamoru begins to fall ill. He tries to keep this from Usagi, but he starts coughing up black blood and inevitably Usagi finds out. Helios appears and introduces himself as Elysion's priest. He tells them that Elysion is a sacred ground that protects the Earth. It exists deep within the Earth. He explains:
The jewel on Helios' forehead shimmers onto Mamoru's chest, revealing a black, decaying rose within. Mamoru looks at it in shock. He shows them that he also bears a black rose within his chest. He tells them that Elysion is contaminated...so both Helios and Mamoru are affected. Next is the Earth...But Helios can do little to help. His body was captured by Nehellenia, so he only appears in the form of an illusion, as either himself or as the Pegasus.
Sailormoon and the others vow to find the Golden Crystal, for it is the only thing that can save all four connected spirits. Later, Pegasus takes Usagi and Mamoru to the ruins of Elysion. Mamoru learns that during the time of Silver Millenium, Mamoru and his family resided there. It was known as the Golden Kingdom. Usagi figures out that Mamoru is the carrier of the Golden Crystal, much like she is the carrier of the ginzuishou.
Helios has deep respect for the prince and princess, and the feeling's mutual. Helios really risked a lot by communicating with Mamoru and the senshi all those times. But he's so devoted to his prince, his kingdom, and his maiden, that he would fight until the end.
For an interesting article on the spelling of Helios' name, please read Ian Miller's article on it at DIES GAUDII.