It's a Kaori Yuki manga. This is all you need to know.A tale of a boy named Setsuna Mudo, a Troubled, but Cute high school student who also happens to be the reincarnated fallen angel Alexiel. Now he's gotten himself in the middle of a war between heaven and hell; only this time, it's the soldiers of heaven who are evil unholy bastards.But this of course, is only the Back Story. The real plot is that Setsuna wants to get in his (biological) sister's pants.At least, that's the case for the anime and the first arc of the manga. Things go on much longer in the manga, and ultimately become quite complicated and aggressively bizarre. Mutated fetuses try to destroy the universe towards the end.Has a series of drama CDs, which is based on several manga arcs.
Always Save the Girl: Played straight at first, then subverted: The first time Setsuna reaches Sara, she refuses to go with him, since it would trigger widespread consequences.
Ambiguous Gender: Belial made a decision when she was an angel to be genderless. Since it was decreed by God that angels must have a sex, the result didn't quite work out as intended.
Attempted Rape: Astaroth threatens to rape Kurai, but he gets stopped by Astarte taking over his body. Early on in the manga, Sara has a flashback of Setsuna protecting her from this as a child.
Body Horror: Plenty of examples, mostly provided by Rosiel.
Brother-Sister Incest: It is even relevant to the plot. Unlike many similar situations, Setsuna and Sara go all the way and even gain a kind of happiness, albeit a very short lived one, as a couple. They are finally together again at the end of the manga. All the angels are related, so their couples including Alexiel/Lucifer and Zaphikel/Anael.
Dude Looks Like a Lady: Rosiel, Arachne, Astaroth... Kaori Yuki seems to love this trope.
To give a specific instance, when some mooks wanted to kidnap him in the middle of the night, Rosiel was able to convince them that he was woman just by talking in falsetto. The mook's reasoning is paraphrased as thus: she was way too pretty to be a man and so hence, is not Rosiel.
Dying Moment of Awesome: Kato is really fond of this trope. He has THREE, the last one being permanent.
Empathic Weapon: An interesting double case. Alexiel's sword, Nanatsusaya, houses a spirit which serves and follows her because she knows his real identity (Lucifer). On one of their adventures, said spirit's human form finds a katana with spiritual powers and sentience, which asks him to follow him around. So, the mystic sword's personified spirit can now summon a mystic sword of his own. Is that a good weapon or what?
Fate Worse than Death: No, not being caught in a some endless cycle of reincarnation of cruel lives and brutal deaths, each time. Being imprisoned by God and slowly fed to your children (no surprise angels are as messed up as they are) is probably a lot less fun, isn't it Adam?
Rosiel didn't have it easy either. Seems that God enjoyed torturing his closest family the most.
Light is Not Good: Nope. Not at all. Conversely, Dark Is Not Evil. Hell, one of the most sympathetic groups in the manga are a race of underdog demons called the 'Evils'.
Lotus-Eater Machine: Setsuna gets trapped in very early in the manga. In this dream, Sara is not his sister, but his girlfriend, he is a normal teenager, and the guys who died in the first volumes are alive and well.
Murder the Hypotenuse: Laila set up her rival to be killed by their loved one. Which he regrets all of his life.
Not Blood Siblings: Subverted even before the series got going. One of the first scenes in the OVA is Setsuna resigning himself to the fact that he and Sara are related.
No Woman's Land: Heaven is this. While most men there do not have it easy, women have it worse.
Winged Humanoid: Almost everyone, which is not surprising. Some characters have unusual numbers. Adam Kadamon has 6 wings, Rosiel and Alexiel have 3, and Lucifer has four. This is supposed to be a sign of their power.
In angelology, some of the higher castes of angels are supposed to have more wings than the lower castes. The idea is overall sound, but it isn't actually accurate. Assuming you believe the really straight angelology stance on this, Lucifer should have 6 wings because he was a Seraphim.