First of all, the name "Hevn" is merely a codename for this buxom and enterprising young woman who is at times an ally to the Get Backers - and sometimes at odds with them. Hevn is a "chuukaiya" (仲介屋) or negotiator, a person who mediates jobs between high-paying clients and retrievers (such as the Getbackers or Shido Fuyuki). She hangs out at the Honky Tonk cafe, and shares a good working relationship with Paul Wan, its owner. She's also fond of Ginji Amano, like most characters in the series. Similarly, like many of these characters, she dislikes his partner, Ban Mido.
Due to her status as a non-combatant, she's a little afraid of getting into fights. However, in the Divine Design arc, she was used effectively by Maria Noches to interfere with Lucifer's plans.
In the Manga
Here, she is most known for her excessively revealing outfits which usually reveals her extremely large breasts (which Ban tends to take advantage of to cop a feel) and her sly, working-woman attitude. She's also fond of trading barbs with Himiko Kudo about her lack of figure - but in reality, Hevn respects Himiko for her spunk and professionalism and their catfights merely reflect their competitiveness as two very capable women working in the dangerous underground.
In the Kiryuudo arc, she tags along as a spectator, after the helicopter she was in (along with Ginji and Kuroudou Akabane) crashed.
It has been hinted that Hevn may know Masaki Kurusu, one of the Four Kings of VOLTs. In book 30, she requests Akabane to transport her to the top of Babylon Tower, after her first request of asking Akabane to transport the GB duo to the same place was turned down. (Akabane cited conflicting interests as his reason for turning down the first quest.) As Dr. Jackal left her, she wondered about the true nature of the "Lost Time", and hoped to see a man named Masaki at Babylon Tower, if he is indeed there.
As it turned out, Hevn and Masaki were lovers. Although Masaki tried to forget his feelings for Hevn (and tried to kill her as well), in the end, the couple embraced after Masaki's fight with Ginji.
In the finale, when Ban tried to do his usual cop of her bust, Masaki grabbed his hand in time.
In the Anime
Here, she is gratuitously more clothed, although she is still not entirely decent. Also, her background is quite different from the manga version (This change also occurred to many characters who made their appearances in the manga as well, for their background information were not known when the anime was produced.). Episode 41 of the anime involves Hevn's background.