Mercer Frey

Mercer Frey is a Nord thief and the leader of the Thieves Guild in Riften. He took over the position of Guild leader after his predecessor, Gallus, who was supposedly murdered by Karliah, Mercer's former partner.

Glitch
(No longer works on PC) When you obtain both Eyes of the Falmer from Mercer, you can bring them back to the Thieves Guild and sell them to the fence, with the base price of 2500 septims for each eye. After you sell both eyes to the fence, go talk to Delvin; you will have a Speech option regarding the Eyes of Falmer. Select that dialogue and you will recieve around 900 coins even though you already sold them. (The Fence has her price set upward 6000 coins for the Eyes once she has them)

Sometimes when downed during Speaking With Silence he will not stand back up unless injured, then he will not open the door at the end of the quest.

He apparently is a vampire or has some form of Vampirism due to him using Vampiric Drain during the quest Speaking With Silence on enemies.