In Dragon Age 2, there are several optional party members that can join you throughout the game.
Sebastian is a Rogue that is only available in The Exiled Prince downloadable content, and he is an optional party member that will not join you automatically during the main quests.
To have Sebastian join you, you need to do the Duty quest which is available in either Act 1 or Act 2 in Hightown. After you’ve done the Duty quest, the Repentance quest will be available only in Act 2 in Hightown as well. After you complete the Repentance quest, Sebastian will join your party.
Sebastian’s third companion quest called Faith will also be available in Act 3 after you’ve completed the first two.