Sometime during the game (it does not have to be after saving/killing the boy in Redcliffe), there will be a cutscene of a mysterious fellow being told to assassinate you. You will encounter him as you travel from place to place and it will start out with a lady telling you to help her with her cart. You will then be attacked by many assassins and Zevran. You can add him to your party after the fight.
(In some cases, you may not get a cutscene of him being told to assassinate you, this happened on one of my characters)