Jacen is a Dalish elf prisoner who is kept in a guardhouse cell in the City of Amaranthine.
Involvement
The guardhouse can be reached only by the Warden-Commander during the final part of the Smuggler's Run side-quest. Assassinating the Lieutenant of the City Guard will provide the key to open up Jacen's cell. The key can also be stolen.
Once the cell is open, Jacen will recognize and praise the Commander and he will offer his archery skills as a reward for the latter's rescue. If the Warden-Commander agrees to it, Jacen can participate in The Siege of Vigil's Keep by sniping targets.
Note: If he's not recruited, Sergeant Maverlies will offer the same function.
If the Commander refuses Jacen's offer, the latter will simply bid them farewell and escape the guardhouse.