Kohaku is a rather accident-prone angel who has come to Earth in search of the Angel Master, Hisui. After getting trapped in a tree and attacked by a crow, Kohaku is rescued by Shuuichirou, a young doctor. As a result, Kohaku feels indebted to grant him one wish but finds the kind doctor is satisfied with all aspects of his life. Thus, Kohaku remains with Shuuichirou in hopes of repaying him somehow (subsequently getting in other sticky situations as a result). Kohaku is neither boy or girl. Angels and demons do not have human genders, so referring to Kohaku as "he" or "she" is incorrect. However, the English translation of the manga took liberty with this and assigned Kohaku female pronouns.

Wish spoilers
After Shuuichirou's sudden death, Kohaku is put into a hundred years sleep, so that Shuuichirou will have been reincarnated by the time Kohaku awakens. One hundred years later, the reincarnation of Shuuichirou, now 17 year old boy named Kudou Shuishirou, passes by Shuuichirou's old house which awakens Kohaku. He has no memory of Kohaku, so their relationship starts anew.
Kobato spoilers
Kohaku is an acquaintance of Genko and several of the inhabitants of the Spirit World, many who admire the angel's singing voice, and has commented on Kobato's progress and the consequences of Ioryogi's actions. After meeting the angel, Ginsei begins visiting Kohaku on a regular basis, particularly as he begins to worry about Kobato and Ioryogi's fates if Kobato is unable to fulfill her wish.

In the manga's epilogue, Kobato and Kohaku meets for the first time in Genko's shop and sing a song together in front of Ginsei.

Kohaku lives with the current reincarnation of Shuuichirou and tries to help him in every way possible.
In the anime series, Kohaku is introduced early in the series and befriends Kobato. Because Ioryogi is aware of Kohaku's true nature, he constantly asks Kohaku if there's something they can do to change Kobato's wish, though Kohaku's response is always that there's nothing to do. At the end of the series, it is Kohaku who explains to Fujimoto the nature of Kobato's wish and assures the young man that if they are fated to be together they will be in time.

Drug&Drop spoilers
Kohaku appears in the latest chapters of Drug&Drop. Kazahaya goes to Shuuichirou's house for an errand and his presence causes Kohaku to awaken. They quickly become friends and promise to meet again. Although it's not confirmed that they're related, Kazahaya and Shuuichirou share the same surname.
(Source: Wikipedia)