Registrar Harlen is the Last Name Merchant for the forces of Order. Located in Altdorf near the entrance to the Emperor's Palace (just to the right and around the corner from the Flight Master off the Market Square), Order players of level 20 or higher can speak to Registrar Harlen, pay 4 gold and add a surname (last name) to their character name. The full character name is only displayed as text above the actual character avatar in-game (not in the target or party frames), as well as in the character selection screen before login.
Registrar Harlen is located in Altdorf. Once you leave the Main Flight area, turn right and though turn right under the archway, once though the arch, turn right again and up the flight of stairs in front of you and he is just on your right hand side.
Registrar Harlen also is a waypoint in the Spotting Trouble quest in Altdorf. After you have inspected the floating corpse at the end of the docks, you are told to talk to him and bring him the family crest you found on the corpse. After completing that part of the quest by contacting him, you must then find a book that he has lost somewhere in the courtyard nearby. It can be found on the little landing from the stairs opposite the landing on which Harlen stands.