Etienne Rarnis

 Is the male prisoner in Diplomatic Immunity. If you choose to speak with Etienne and are a member of the Thieves' Guild, he will belatedly remember seeing you around the Ragged Flagon. Upon releasing Rarnis, it is possible for him to be killed by a Frost Troll in the caverns below the Thalmor embassy, as he charges in unarmed and unarmored. However, if you slay the troll quickly enough, Etienne will be grateful, commenting "you didn't have to let me go back there, so.... thanks." If he survives, he can be found in the Ragged Flagon Cistern, and will be glad to see that the Dragonborn has survived, also promising that he'll never forget his savior from his Thalmor captors.

He was captured by the Aldmeri Dominion and then interrogated and was claimed to be one of the Blades' Loremasters prior to the first war against the Empire. He was not a field agent, but was believed to have been behind one of the most damaging operations carried out by the Blades against the Aldmeri Dominion.