The Aliens were Alan and Ann, who were lovers. Their main purpose was to steal energy from humans with their monsters: cardians. The cardian would steal the energy and give it to the Doom Tree, which in turn would give Alan and Ann energy.
To summon a cardian, Alan would play his reed flute.
Alan
As an alien, Alan was the more practical of the two, thinking
before acting. He would play his reed flute to summon a cardian,
and almost always let Ann pick the cardian.
As a human, his name was Alan Granger. He always was followed around constantly by girls for his
good looks. When he first saw Serena, he immediately fell in
love with her and vowed to make her his.
Ann
As an alien, Ann was the more aggressive of the two, usually acting
before thinking. She picked the cardian for Alan usually.
Ann went to Serena's school as a human - names Ann Granger, and was in her class. The first
time she saw Darien, she immediately fell in love with him, and constantly
competed with Serena to win his heart. Serena tried to become friends
with Ann also, but Ann rebuked all her attempts.
Later, they learned from the Doom Tree that they were the children
of it.
When Sailor Moon healed the Doom Tree and turned it into a seedling, they became good
and left the earth, wanting to start a new life with their newly
healed seedling.