The Avenger was one of the Kurdish prisoners whom Dante was tasked with guarding at Acre during The Crusades. He was based upon the assassin (Hashashin) whom the original Dante mentioned in the 19th canto of the Inferno named "lo perfido assassin" (The treacherous assassin)
In exchange for his freedom as well as her own, the man's wife who claimed to be his sister offered to "comfort" Dante. Dante took her offer which only further enraged the man. After the Siege of Acre, he travelled to Dante's villa in Florence where he assaulted and killed both Alighiero and Beatrice. In the Hall of Gluttons, Dante learned that the Avenger was the Slave Girl's cheated husband, not her brother when Lucifer made him witness Beatrice's murder.
He was seen again in the downloadable prequel, Dark Forest. When Dante apprehended him, he repeated his line "She was not my sister! She was my wife!" only this time the demons transform the assassin into a dog/goat/fox like dijin with swords for hands riding a hybrid of a horse, a ostrich and a Mongolian death worm. Although portrayed as a villain the avenger is nothing more than a tragic man demanding justice and nothing more