Alan is normally a very calm, collected, and rather introverted individual. In his human guise, his mystique draws the attention of lots of girls in school, and his mystique is enhanced by the flute he always carries with him.
This flute is his means of summoning creatures called Cardians from a deck of cards he also possesses.
But then Alan saw and then fell for Serena. He began making romantic advances, taking the patient, gentlemanly approach (in contrast to Ann's more assertive approaches with Darien), but though Alan is cute, Serena at that point is already focused on restoring Darien's memories, leaving Alan with a very bitter taste in his mouth. He would've squashed Darien...but Ann had fallen for him, making the whole business extremely complicated.
But it's Alan who realizes that there is a strong bond between Serena and Darien, one the two of them cannot break because each is ready to protect the other. With that, he orders the Doom Tree to drain both of them, but the tree lashes out instead, aiming for Alan...when Ann takes the deadly blow for him. Seeing her die in his arms makes him realize just how much he cared for her...too late. He sat there, ready for the blow that will join them both in death...
But Sailor Moon manages to snap the tree back to reality, and with its waning energy, the tree tells Alan all it could about their shared past, about how he and the rest of his race were borne from the tree but then tainted by darkness.
When Sailor Moon destroys most of the Doom Tree, a little seedling remains, and what love energy it had flows into Ann. With that, the darkness that had tainted them had been cleansed. The two aliens, along with the new seedling, leave to start a new life elsewhere.