Hexxat

Hexxat is a vampire thief and the fourth extra companion in Baldur's Gate II: Enhanced Edition. She is also the only undead companion available. She can be found in the Copper Coronet in the Athkatla Slums upon first entering it. If she is not recruited her before Chapter 4, she will no longer be at the tavern, and you will have missed your chance to talk with her.

In-game biography
"A striking woman of Chultan descent, Hexxat holds her secrets dear. When pressed, she speaks in vague terms about her mother, a cleric of Ubtao, and the aunts who raised her. Of her life beyond childhood, she says nothing, other then that her immediate assignment was given her by someone known only as L. Discretion is a requirement of Hexxat's profession, and she prides herself of being a consummate professional."

Related quest

 * Hexxat's request