Finn Fish is the angel, or semi-angel (jun-tenshi), who helps Maron detect demons and is also the one who transforms Maron into Kaito Jeanne.

Spoilers for Manga and Anime on how the series ends:

As the story goes on, it's revealed Finn had been manipulating Maron the whole time to gather chess pieces to deliver to the Devil. The former half-angel reveals herself as a fallen angel named Da-Tenshi Finn Fish. She was in love with the Devil. In the manga, she is reborn as Natsuki Nagoya, Chiaki and Maron's daughter.

The manga and anime versions of how Finn became a Fallen Angel are very different. In the anime, she is on her way to Earth when she in kidnapped by the devil. He brainwashes her, but then wipes her memory and turns her back in a semi angel so that Maron wouldn't be suspicious. She is brought back to her true self by Maron in episode 44. In the manga however, she went voluntarily to Lucifer. As one of the first three dark angels to become a semi angel, she was allowed to go to Earth with the other two angels, Celcia and Toki. There, Celcia was injured and take in by a priest named Sagami Kugahara (name is in english order).

Misunderstanding the situation, Finn and Toki attacked Sagami, but as they were in their tiny forms, they only succeeded in cutting his finger. He brought them to Celcia, and Finn used a strand of her hair to fill her friend with holy energy. They then explain to Sagami how holy energy in stored in their hair. In an effort to pay Sagami back, Finn went to meet him in cave, ignoring Toki's pleas not to go, and even runnng away after he confessed that he had always loved her. When she met him, she saw a girl frozen in ice, who (through a picture on Sagami's desk) had previously been identified as "Natsuki", Sagami's sickly sister who died of an illness. As Finn angrily explains, this was not true, and Sagami had killed Natsuki in order to freeze her so the holy ice would melt faster, providing more holy water to be sold. She reveals herself to be Natsuki, calling Sagami "Nii-San" (big brother). He captures her and cuts her green hair, which he explains has far more power than even the whitest hair (believed to store the most holy energy), in order to dip it into the holy water and make it even stronger. However, her hair had the power of a full angel, and exploded, killing many people. She was tried for murder in heaven and sentenced to going through the Gate of Annihilation. However, the devil gets to her first, and she goes to him and becomes his servant.

It is later explained (in volume 7) that the reason was because of a promise she made with Access before she went to Earth. She promised to listen to what he was going to tell her (he was going to tell her that he had always loved her) and chose to go with the Devil to become a Fallen Angel so that she would be able to meet Access again. In the end, she sacrifies herself so that Maron survives the battle with the Devil. When she died, Access gave her his earring, and was reborn with the same earring in her hand.

Finn is referred to as "Queen" by Myst and Noin in the anime and are shown far more respect then in the manga, where Silk punchs Finn because she threw the cross Maron used to change into Jeanne off the balcony, leading to Maron jumping off trying to get it. Finn is referred to as Finn in the manga and anime though in the anime Access calls her "Finn-Chan". Fin loved Access after that!

(Source: Wikipedia)