Earthian (アーシアン, Āshian) is a shōnen-ai manga by Yun Kouga made into a J.C.Staff-produced anime OVA. There are four OVAs total, three of which continue the same plot, and another that delves into a separate side story.
For thousands of years angels have observed the behavior of humans from their homeland Eden. They are disgusted with humans for how they treat their planet and how they treat each other. It is decided that the humans should be wiped out. However, to give the humans a chance, Eden sends down many pairs of angels to observe the humans. One is a plus checker, who seeks the good in humans; the other is a minus checker, who seeks the bad in humans. If more good is seen in humans, they will be spared. But if 10,000 minus checks are made against them, then they will be destroyed.
Along with humans, angels fear and despise Lucifers, also known as fallen angels. An angel is typically born with light hair and white wings, but if an angel acts against Eden too many times, then their hair and wings will turn black, known as the Black Cancer, and they die. To encourage that angels stay away from both humans and Lucifers, it's decreed that if an angel exposes their wings to three humans or if they meet too many Lucifers, they shall also become one as well.
Chihaya is an orphaned angel of Eden. He was born with black hair and black wings, resembling a Lucifer. Because of this, he was shunned for most of his life, finding acceptance only in the residents at the orphanage and his adopted father Michael. He becomes a plus checker to see Earth and because of his love for humanity. Over time, he continuously interacts with Lucifers and feels a kinship towards them because of their hair and wings.
He is paired with Kagetsuya, a handsome and popular white-winged angel who hates mankind and holds hostility for Lucifers. Due to this prejudice, Kagetsuya is extremely hostile towards Chihaya at first, constantly belittling him because of his feelings and appearance. Despite all this, Kagetsuya often finds himself carried away with Chihaya's ideals and constantly aiding him, though it's against his position.
Things grow even more complicated for the pair when they fall in love with each other. Homosexuality is forbidden in Eden, but that doesn't stop their feelings for one another. Along the way, many secrets are discovered within Eden itself and the angels and the pair, especially Kagetsuya, must decide which side they will choose.