Sarevok Anchev is the main character's half-brother, who appears as a ghost near the beginning of the content of Throne of Bhaal. He helps the player character exit the Pocket Plane once he receives a piece of the soul of either the player character or Imoen, thereby bringing him back to life. After showing the player character the way out, he offers his services. Although his previous death has stripped him of all of his Bhaalspawn powers, he still remains a formidable melee fighter. The player can change Sarevok's alignment to Chaotic Good by setting a good example, and forgiving Sarevok for his past transgressions.
Source: Wikipedia, "List of Baldur's Gate characters", available under the CC-BY-SA License.