Mercer Frey

Mercer Frey is the villain of the Thieves' Guild story in The Elder Scrolls V: Skyrim. He is the leader of the struggling crime ring who is very authoritatively mean to the player when they first join and blames a woman named Karliah for murdering the previous leader, Gallus, who he took over from, and causing the downfall of the guild. In reality however, Mercer killed Gallus and blamed Karliah for it. This is revealed when the player goes on a mission with him to find and kill Karliah, only to be betrayed by Mercer, who tries to kill the player. Karliah saves the player however, and they embark on a quest to clear her name, reveal Mercer's true nature to the rest of the guild and stop him. It is revealed that Mercer, Gallus and Karliah were all "Nightingales", agents of the Daedra prince(ss) Nocturnal, and that Mercer betrayed the dark goddess by stealing her "Skeleton Key", which he has been using to steal from the Thieves' Guild, which is the real reason it is now bankrupt. Eventually, Nocturnal herself orders the player, Karliah and fellow thief Brynjolf (who becomes a Nightingale as well) to stop Mercer Frey once and for all, and he is fought and killed in the Dwemer ruin of Irkngthand.

When confronted, Mercer claims that the player and the other thieves are no better than him, as he believes that "honor among thieves" is an illusion and that, if one is to be a criminal, they shouldn't have any moral inhibitions.