He took over the position of Guild leader after his predecessor Gallus was supposedly murdered by Karliah, Mercer's former partner, until the Dragonborn discovered the truth while looking for Karliah, with Mercer in tow in the ruins of Snow Veil Sanctum.
During the Dragonborn's initiation to the Guild, Mercer is usually the one that gives out tasks for them. It is noticeable from the beginning that he always acts impatient and irritated towards the Dragonborn. He obviously doesn't like that they joined the guild, since he doesn't want to have anyone to dig in its problems because of his previous actions. He eventually has enough of the Dragonborn, and tries to get rid of them during Speaking With Silence.
- Main article: Multiple Followers
Mercer Frey is one of the quest characters that can be exploited to follow the Dragonborn throughout Skyrim along with a follower of their choice. In the quest Speaking With Silence, after they enter Snow Veil Sanctum, they can leave and he will continue to follow them.
This is a simple way to obtain a powerful follower as, whilst the mission is still available, he cannot die. While it is not possible to converse or trade objects with him, he will remain an unkillable ally; at least until Karliah is found.
Mercer is fairly pessimistic, stressed, and irritable. Despite his fairly irritable personality, he appears to be quite sly, for example killing Gallus and blaming it on Karliah, and using Karliah to lure the Dragonborn to Snow Veil Sanctum. He also seems to be adept in swordsmanship, as Karliah states when talking to Mercer, "Crossing blades with you would be a death wish, Mercer." He seems to be a master of sneak as he was able to silently approach Falmer and sleeping Bandits and assassinate them swiftly while also sneaking past many of their Chaurus. This could have potentially been his obtained hidden potential from the Skeleton key as he is seen teleporting in the fight with him. His lockpicking skill is debatable: he is seen picking locks that are unpickable, but this is almost certainly because he had the Skeleton Key in his possession, and it is possible that he has generally good knowledge of the locks themselves.
The shelving behind Mercer's desk in the Cistern slowly fills with trophies relevant to any exploits and larceny targets sold to Delvin Mallory, such as the Left Eye of the Falmer. Upon finishing additional jobs Delvin and Vex give the Dragonborn, trophies of recovered items begin to fill the shelves too:
- Jeweled Candlesticks appear after completing 5 jobs.
- An Ornate Drinking Horn after completing 15 jobs.
- A golden ship model appears after completing 25 jobs.
- A golden urn appears after completing 35 jobs.
- A jeweled goblet appears after completing 45 jobs.
- A jeweled pitcher appears after completing 55 jobs.
- A jeweled flagon appears after completing 75 jobs.
- After 125 jobs, a safe appears along the safe wall, next to the bookcases. It contains gems, gold and some thief potions.
- If the Thieves Guild Quest Darkness Returns is completed, a shrine of Nocturnal appears in the Cistern. This bestows a sneak blessing.
- After becoming the Guild Master, a chest appears in front of the desk in the Cistern. It contains gold and gems and is periodically restocked.
- Mercer Frey is voiced by Stephen Russell, who also voices another master thief, Garrett, from the Thief series.
- Mercer's signature weapon is a Dwarven Sword of Devouring, which he supposedly stole from Irkngthand.
- The High Elf Kornalus shares his surname, but they have no relation whatsoever.
- During the final fight with him, a Dragonborn with a high Sneak skill can hide from him, causing him to proclaim "The shadows are my domain!" This happens during any combat situation.
- During the quest Blindsighted Mercer Frey says "The die is cast" (Alea iacta est). This means the point of no return. Also it could mean that once the fight with Mercer Frey has started there is no going back.
- He will dance and drink just like a normal character would around a bard when one is playing a song. This makes him rather funny to look at given his taciturn nature.
- The surname "Frey" may be a reference to House Frey from George R.R Martin's A Song of Ice and Fire series.
- (No longer works on PC) When both Eyes of the Falmer are obtained from Mercer, it is possible to bring them back to the Thieves Guild and sell them to the fence, with the base price of 2,500 septims for each eye. After both eyes are sold to Tonilia, go talk to Delvin; there will be a speech option regarding the Eyes of Falmer. Select that dialogue and around 900 coins will be received even though the Eyes have already been sold. (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.
- If Mercer is brought to the College of Winterhold while the unbound dremora from the Conjuration Ritual Spell quest is still around, he will openly attack the dremora but will unable to do any damage to him for some reason. Also since Mercer will be openly hostile towards what is technically a resident of the College of Winterhold, all the other Mage residents will become hostile towards only Mercer, which results in a never-ending fight.
- Mercer can sneak roll, but he still sets off tripwire and pressure plates even though for the Silent Roll perk the perk Light Foot is required.
- During the quest Loud and Clear, obtained from Brynjolf when joining the Thieves Guild, if the Dragonborn does not speak to Brynjolf before entering the Ragged Flagon - Cistern, the Dragonborn is able to attack Mercer Frey repeatedly and he nor anyone else will become hostile.
- In Speaking With Silence Mercer may not open the first door into Snow Veil Sanctum. The problem may be circumvented by telling all followers (who can be told) to "wait here," at which point approaching the door should get Mercer to walk down the stairs, utter a comment about the ruins being impenetrable, and move forward to unlock the door. The problem occurs because no followers may enter the ruins, aside from Mercer.