Cullen was originally apart of the Templars who reside in Ferelden, overlooking the mages in the Circle of Magi. After the uprising brought forth by Uldred, forcing Cullen to watch many of his comrades be slaughtered by abominations, Cullen was sent to Kirkwall under the orders of Knight Commander Greagoir. His mental scarring through this ordeal left him incredibly distrustful of mages, and increased his cynicism upon the matter regarding mages and blood magic. Greagoir was privy to this development, and felt Cullen was becoming too harsh of a Templar towards his watches and would be better suited at Kirkwall, to which is governed by a stricter set of rules over the mages, and required the help.
In both Dragon Age: Origins and Dragon Age 2, Cullen is voiced by Greg Ellis.
Cullen can be first introduced to the player earlier on than usual, should the Magi origin be selected in the character creation menu. Here he is introduced as a fairly new member, and one who sympathises towards mages more so than most. He was also assigned as the Templar to cut the Magi Origin Warden down, should s/he have failed the Harrowing (a test of will amidst the fade to see if they withstand the temptation of demons) and became an abomination. Should the Warden mage also have been a female, it is strongly implied that Cullen has feelings towards the player.
No matter what origin is selected, Cullen will always be found amidst the chaos once the Warden heads (or returns) to the Circle of Magi in Ferelden. One of the Senior Enchanters, Uldred, had rallied the mages to rise up against the Templars, in hope that Loghain would save them and give them their freedom. A great many of Uldred's followers resorted to Blood Magic, however, and Uldred himself became an abomination of Pride, and his once somewhat honourable intentions were twisted into the dire need of hunger and destruction.
Cullen is found by the Warden nearing the top of Tower of Magi - captured, disoriented and barely making any sense. He is greatly distressed and his once sympathetic view of mages has now switched to a seething hatred. His trust of mages was completely abolished and his paranoia bled into his vision making him believe that virtually all mages were blood mages. No matter how the Warden interacts with him, Cullen will always be tainted and hallowed by this experience. He will also always survive no matter what transpires through the Tower. Regardless of whether the Warden calls upon the Templars or the mages for aid against the Darkspawn Blight, Greagoir will see it fit to keep Cullen behind because of the ordeal he suffered.
After the blight is ended by the Warden, Cullen is ported to the Free Marches up north of Ferelden to the city of Kirkwall. His own experiences towards mages lead him to becoming far too harsh of a watcher, and Greagoir thought it best to keep him away from the Ferelden mages. Kirkwall, being one of the most controversial Circles, was the perfect place for Cullen to carry on his Templar career. His own opinions towards mages also greatly meshed with the Knight Commander of Kirkwall, Meredith, and as such Cullen found himself promoted rather quickly to Knight Captain.
Cullen is first brought into the Dragon Age 2 storyline in Act 2, when Hawke is investigating the rumors about some rather suspect methods of training for Templar recruits. He is found arguing with a Templar recruit, only to find that recruit to be possessed by a demon similar to a mage. Hawke manages to help out Cullen and helps with the investigation about blood mages attempting to infect Templars with demons. It is soon enough found to turn out true, that blood mages are infecting Templars with abominations through blood magic. One recruit, Keran, was undergoing the ritual before Hawke manages to save him. He is shown to not have any demon influences upon or within him, though he can still be removed from the Templar order under suspicion should Hawke coerce Cullen into believing so.
Cullen doesn't play much of a focal part within the story until the very end, though is still involved in a few side quests; such as one where Hawke's friend and Captain of the Kirkwall Guard, Aveline, is accused of ''coddling her troops'', insinuating that the Templars will need to take authority over Kirkwall instead.
Nearing the end of Dragon Age 2 when the outbreak between the Templars and Mages begin, Cullen is naturally on the side of the Templars. Should Hawke side with the mages, however, Cullen will still not be faced in battle. No matter who Hawke allied with, Meredith will still eventually press the attack against Hawke with Cullen objecting, and proclaiming that they were supposed to only arrest Hawke, not kill. Meredith, who was steadily being driven to madness, will then accuse Cullen of being controlled by blood magic, along with the rest of the Templars. She draws her large broad sword, to which is made of a strange kind of lyrium that Hawke found years before, and attacks Hawke and his/her team. At this point, it is clear that Meredith has completely lost it, which forces an alliance of all the characters Hawke has met across his/her's life in Kirkwall - including Cullen.
Once Meredith has been killed, Cullen and the rest of the Templars stand down and kneel in honor for saving Kirkwall from the power-crazed Meredith.