Alan is one of the keepers of the Doom Tree. He shows up along with Ann right after the defeat of Queen Beryl, forcing Luna to re-awaken Serena and eventually the other scouts (they all had no memories of the Scouts after the last fight with Beryl). He seeks energy from the Earth to keep the Doom Tree alive. Without the Doom tree (a parasite that kills planets) Alan and Ann will both perish from lack of energy.
Alan poses as a student (Alan Granger) at Serena's school along with his lover Ann. To the humans they are brother and sister. Alan takes an immediate liking to Serena and Ann gets very upset... that is until she sees Darien. While gathering energy he also pursues Serena, and tries to win her over.
This is Alan (blue hair) and Ann (pink hair) in their true form: