Cahir is a Chasind mercenary. He is Duke Prosper's personal bodyguard.
Involvement
In Mark of the Assassin, Cahir is first seen beside Prosper during Hawke's arrival at Chateau Haine. Hawke recognises him as a Chasind and can then express his/her opinions on the Chasind as a people; all choices prompt an amused response from Prosper, but Cahir is unfazed regardless.
When Hawke is in the Chateau Haine dungeons, there is the choice to escape by traveling through a tunnel system under the mountain or fighting through the chateau. If Hawke chooses to go up the stairs into the Vaults, Duke Prosper ambushes the Champion and leaves Cahir to eliminate him/her. Cahir, irritated at being abandoned, then leaves the fight to Prosper's soldiers to unleash the "pets." After this, Hawke is forced to continue on through the tunnels.
In the caves, Cahir traps Hawke and his companions while separating Tallis from the group. He is accompanied by his "pets," a group of crazed mages he claims responsibility in having broken. When Hawke queries why he would fight for a man who abandoned him to face them, Cahir explains that in fact he cares nothing for Prosper, or the duke's enemies; he prefers to have more interesting fights against capable opponents of his own, such as Hawke. The Chasind then attacks and is slain, allowing the party to travel through the tunnels freely.
Drops
Money 86 
, 18 
Portcullis Key
Dragon's Breath