Dragon Age: Origins
Tamlen belongs to the same Clan as The Warden (if Dalish) and Merrill, and is a temporary companion in the Dalish origin. While hunting with The Warden, the pair come across several humans who believed they had found ancient ruins in a nearby cave. After dealing with them, the two investigate the humans' claim, finding ancient elven ruins inside the cave. Searching further, they locate a mirror. As Tamlen peers into it, he sees images flashing inside, curious, he touches the mirror and vanishes. His companion, The Warden faints, and is eventually rescued. They later return to find Tamlen, but his fate is unknown.
It isn't until much later, during a Darkspawn attack on your party's camp, that you discover Tamlen's ultimate fate. Having been corrupted by the mirror's power, he has begun his descent into turning into a ghoul. Retaining memories from his past, he flees from The Warden. When followed, he begs them to end his suffering.
If The Warden is female, her relationship with Tamlen is implied to much more then friendship.
Dragon Age 2
During Merrill's companion quest in Act 3, ghosts of Tamlen and other Dalish elves attack her and the party. He warns her of the Eluvian, and that her curiousity with it could be disastrous.