Race: HumanCullen's Commander, and the one who assigns him to be with the player-character mage (in their origin story) in the harrowing chamber and to be the one to kill the player-character if they fail and turn into an abomination.
First meeting: Dragon Age: Origins
When the Warden arrives at Kinloch Hold seeking help for the Fifth Blight, Uldred has attempted to take over the tower by using blood magic and setting demons loose in the tower, and Greagoir has the tower in lockdown and has sent for permission to invoke the Rite of Annulment to destroy all the mages in the tower.
While very stern and gruff, Greagoir has respect for senior mages such as Senior Enchanter Wynne and First Enchanter Irving, and can be talked down from annulling the circle if the Warden is able to clear the tower of demons and if Irving survives. It is up to the Warden how they wish to handle the tower from that point forward. Depending on how the Broken Circle quest ends, the Warden will have either Templars or Mages in their army when they go to fight the Archdemon.
Greagoir does not appear in Dragon Age 2. It is unknown at this time whether he will appear in Dragon Age: Inquisition.
For more in-depth information, see the following: