Courier (Skyrim)

Couriers bring messages to the player, often activating certain quests. They can also be found with messages to other people and will tell the player that he is in a hurry.

Oblivion
In, a female Imperial is named Courier, though a specific name is not given. She rides a Black Horse around Cyrodiil, delivering the copies of the Black Horse Courier. She hands out a random copy from the Black Horse Courier every time you speak to her. She is usually found riding on the roads of Cyrodiil, but sometimes in the woods or somewhere further running away from an enemy.

Skyrim

 * In Skyrim, a courier brings the Dragonborn a letter for the first time after he or she uses a Dragon shout in virtually any location. This activates a Radiant Story quest, in which the Dragonborn learns the locations of new Word Walls.
 * Couriers also bear Letter of Inheritance when someone close to the Dragonborn dies. Often these letters are accompanied by various sums of.
 * After killing Grelod the Kind for Aventus Aretino, a courier brings a Mysterious Note to the Dragonborn from the Dark Brotherhood.
 * Couriers can also bring letters when the player acquires certain enchanted weapons or armor from vendors or from smithing/enchanting them. These quests usually ask the Dragonborn to deliver said weapon to a rich researcher who offers suitable reward for bringing him the item. A good example is the Letter from Calcemo.
 * Courier letters trigger follow-up quests for The Man Who Cried Wolf and The White Phial, respectively.
 * Elisif the Fair and Falk Firebeard will both send a letter by courier to the Dragonborn.
 * Letters revealing the locations of Word Walls can be stolen from the couriers. This can sometimes result in them sending Hired Thugs after the player.
 * Couriers have randomly-generated clothes and a hat. They will sometimes not have any clothes at all.
 * Couriers may be found in the wild, e.g., a Stormcloak courier. They run to an area and respond generically when talked to. They can also try to get the player to join whatever faction they are affiliated with.
 * Couriers cannot be killed, do not retaliate, and do not incur a bounty if you attack them in public; instead, they draw their weapon and flee. If the Dragonborn attacks a Courier in a public place, passersby will sometimes attack him as well. The attacks will continue, even after they stand back up. (Potential bug in which the courier can be killed with a bow and arrow in Solitude)

Trivia

 * A Courier can be found in the Bannered Mare in Whiterun, warming his hands by the fire. He cannot be hired and will not deliver anything to the player though, just responding with generic dialogue.
 * Sometimes Couriers will spawn with no clothes.
 * A Courier can be used as a safe, if irregular, storage space for weapons. If you pick a courier's pocket and plant a weapon, then attack him but sheath your blade, he will return the weapon to its sheath. When you next see him, the weapon will still be at his side, and it can be stolen back.

Bugs

 * Rarely when a Courier spawns in a town, their animation fails to load. The Courier will stand in the relaxed A-pose and their dialog captions will show, but the player won't hear them speak. The dialog can be advanced by pressing B (Xbox) but the Courier will remain stuck in pose. When approached they will speak to the player and facial animations will work, but they still won't run body animations even if attacked or shouted at. Entering or exiting buildings doesn't effect their position, but fast travelling or being abducted while sleeping appears to.
 * An imperial courier may run into the Dawnstar Sanctuary get hit by a guardian and run out again.
 * Occasionally a courier will be naked when delivering your message.
 * At least in Whiterun, when the courier is bugged in his display pose (as mentioned above) and the player attacks him, the guards will come running and instead attack the courier. Sometimes, the courier can be ragdolled with Unrelenting Force and he will be stuck in a collapsed heap for a while, but will still inform the player (if interacted with) that he has no mail for him / her.
 * Sometimes the courier will say that he "has something to deliver" but no dialog options will appear, and you won't get anything. The courier will then keep following you until you enter a building. but once entering the main world (or town) again he'll be there again.
 * If you're hiding from guards, the courier might try to come and find you, and then give away your hiding spot.