Casavir is a human Paladin who acts as one of the possible love interests in the game. He is initially encountered as a mysterious stranger who is using guerilla tactics to help defend a location known as Old Owl Well. Much of his backstory, and indeed character development, was deleted, though the dialogue files can still be found using the toolset.
While he was not mentioned in Mask of the Betrayer, Storm of Zehir reveals that he survived the destruction of the Shadow King's lair, albeit badly wounded. It is implied that he has been imprisoned by Luskans.
Source: Wikipedia, "List of Neverwinter Nights characters", available under the CC-BY-SA License.