How interesting. It appears Gallus's history has repeated itself. Karliah has provided me with the means to be rid of you, and this ancient tomb becomes your final resting place. But do you know what intrigues me the most? The fact that this was all possible because of you. Farewell. I'll be certain to give Brynjolf your regards.
~ Mercer Frey to the Dragonborn before he stabs him/her
If I see Frey, I'll pluck his eyes from his skull with my bare hands!
~ Vex, after she discovered about Mercer Frey's betrayal

Mercer Frey is a villain from The Elder Scrolls V: Skyrim. He serves as the main antagonist of the Thieves Guild questline. He was voiced by Stephen Russell, who voiced other characters like Lucan Valerius and Belethor. He is the leader of the struggling crime ring who is very authoritatively mean to the player when they first join and blames a Dark Elf 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 framed Karliah for it. This is revealed when the player goes on a mission with him to the Nordic tomb known as Snow Veil Sanctum to find and kill Karliah, only to be betrayed by Mercer, who attempts 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 Daedric 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. He has a very negative personality, as he is extremely arrogant, impatient, failure-intolerant, power-hungry, and an expert on both manipulation and deception.