Little Darling (2000)

Blurb:
Celestia and Hades—mystical realms inhabited by angels and demons. It is these supernatural beings’ duty to watch over the human realm, making sure we are neither too good nor too bad . . . Kaie is just another demon trying to rise up in the ranks of demon society by working in the human world. Unfortunately, he’s got one major thing going against him: his body, which looks like a man’s but is actually hermaphroditic! Luckily for him it isn’t permanent, and Kaie (with every intention of becoming a splendid man) has waited his entire life for the day when his sex would be decided one way or the other. But his plans hit a roadblock when he unwillingly gains the affections of Daina, the heir to the Celestial throne who happens to suffer from an unusual condition that alters his appearance depending on the time of day. Will Kaie’s dreams of becoming a real man fade when the charming angelic heir is so insistent on making Kaie his wife?

Source: www.anisearch.com/manga/35