When first met at the Bee and Barb, he gives the Dragonborn the quest Promises to Keep, in which the Dragonborn must steal Sibbi Black-Briar's horse Frost and give it to Letrush. This has been arranged between Sibbi and Letrush.
After the quest, depending on which paths were taken, Louis Letrush can be the trigger of some random encounters:
Whether or not Maven was informed about Louis' plan, if the Dragonborn delivered Frost to Letrush, the latter will be encountered along the road later on while being attacked by Hired Thugs. The thug will inform Letrush that "Maven sends her regards" before killing him.
In case Maven Black-Briar was informed about Louis Letrush's plans, the encounter with the hired thug attacking Louis Letrush still occurs. If the Dragonborn kills the thug before Louis can die, he will attack the Dragonborn while voicing his suspicions that the Dragonborn must be the root of all his problems by having told Maven about his plan to steal Frost.
In case the Dragonborn double-crosses Louis Letrush in order to keep Frost, a thug will be sent by Letrush instead to take revenge on the Dragonborn.
Even if he's not dead, this should be the last time Louis Letrush can be found (See reported bugs for exceptions).
If the Dragonborn chooses to keep Frost, Letrush will depart for the Whiterun stables and can be followed the whole way there, winding his way through Skyrim's wilds, occasionally stopping to examine or loot a dead animal, and sometimes or veering off the road and stopping for a few seconds to survey the view before returning to the path.
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If a fight occurs in the Bee and Barb, Louis may become hostile to the player and the other NPCs, including guards, will flee. Since he is essential he will remain aggressive and cannot be killed.
Louis will vanish if the Dragonborn talks to him without accepting the quest. The Dragonborn can proceed by talking with Sibbi Black-Briar about stealing Frost. The quest can still be completed, but the player cannot speak with Letrush until the quest progresses to the point of either handing over Frost or keeping it.
If the Dragonborn attacks Louis before handing Frost off to him, he will never die - every time his health drops to zero, he will beg mercy (during which he cannot be attacked), and then his full health will be restored and he will continue the attack. This can sometimes be fixed by returning to Riften and waiting 24 hours, and then returning to Letrush.
After completion of the quest Promises to Keep, Louis Letrush can sometimes be found duplicated near the Whiterun Stables and running about in the general area between Whiterun and Dustman's Cairn - amusingly, without his horse. Louis will duplicate by one every time you buy a horse from the whiterun stables, sometimes being stuck in the ground. Although permanent, this bug has no known, negative effects except for it being highly annoying at times. This can be fixed on PC by clicking on Louis' clone and using the 'disable' command, using the console.
During the post-completion encounter with Letrush the player will receive a bounty if they attack Letrush before the latter became hostile and attacked the player first.
[PS3/Xbox Fix] After witnessing Louis' assassination by Maven Black Briar's mercenary, killing the duplicates might cause them not to reappear.
Should the player happen to kill someone in his vicinity, he will attack on sight, everytime the Dragonborn is spotted. The same applies to duplicates as well.
When trying to kill one or more of the Louis, they may get into a fight with the guards. Since they are unkillable, the battle may go on forever.
360 The Louis Letrush clones may have a bugged face with constantly flashing eyebrows.
360 Sometimes he can be found outside of Whiterun, standing next to his clone who is half underground. When attacked, the clone will jump out of the ground and both of them will fight you.
PS3 If the player attempts to complete the quest In My Time of Need by siding with Kematu, it is possible that a Letrush clone present at the Whiterun stables will become hostile towards Saadia. This causes all NPC's in the area to attack this particular Letrush, including Saadia herself. Considering the clone cannot be killed, the quest will not be completed, as Saadia will continue to fight, sometimes drawing Kematu himself into the battle, and not continue on to the ambush.
Another duplicate can be found wandering near Knifepoint Mine. Oddly, this duplicate cannot be killed, which if taken advantage of can be an infinite source of combat experience.
Upon traveling to Whiterun via the Horse carriages, (the) Letrush(es) will become hostile and cannot be killed. Running to the Guards should attract his attention into a never ending fight, giving the player the opportunity to escape.
Upon entering Riften, Louis may randomly start attacking the player, making it impossible to start the quest.
Sometimes Letrush will not get on Frost when it is delivered to him. Lethrush will spawn outside of Whiterun next to the player's horse and attempt to ride away on it. He can, however, be kicked off.
Occasionally, Letrush may get "stuck" running on top of Frost, and he will start wildly spinning around the horse upon certain enemy encounters.
During the Quest Battle for Whiterun, the player may hit a Louis Letrush clone in the melee, causing a bounty in The Rift, and the aggressiveness of all clones. They seem to be killable.
If Frost is attacked while the player is about to hop off the horse, he 'rubberbands' back and disappears. Saving before getting off the horse can be a good idea.
During the quest A Daedra's Best Friend, Louis may randomly appear and follow Barbas to the Shrine. Barbas will wait for Louis to catch up.
PS3 Sometimes after completing the quest and giving Frost to Letrush, he will go back to Whiterun and get stuck in the ground in front of the stables while riding the horse. If the player returns to this location, 2 more duplicates will spawn on either side of him while he is still in the ground.