Lug-Nut

 is a Raider who appears and confronts the player as soon as they loot the Naughty Nightwear as part of the unmarked quest Grady's Package. He wears a random raider armor and carries a random weapon (hunting rifle, sawed-off shotgun, or  assault rifle)

Lug-Nut is also mentioned by Sierra Petrovita when asked more about Ronald Laren. She will start talking about how tough Ronald Laren is and will mention Lug-nut and two other raiders. The two unknown raiders were killed by Ronald Laren, who then punched Lug-nut in the face.

Unmarked quest: Grady's Package
Upon your taking the Naughty Nightwear, Lug-Nut will appear and demand that you give it to him.

Family
In the DLC pack Broken Steel it is revealed that Lug-Nut has a brother, Lag-Bolt. While Lug-Nut is a Raider, Lag-Bolt seems to be employed by Talon Company (due to his armor), but this is not to be relied on, as he may have scavenged it somewhere.

Nightwear: Origin and conflicts
In a note to Lag-Bolt, Lug-Nut explains more about the origin of both the Naughty Nightwear and the All-Nighter Nightwear. It appears that Lug-Nut had the two pairs made in Virginia by a man only known as "The Tailor". It is obvious that they were made to be put to use under sexual circumstances, due to the character of the letter.

However, the questions how Ronald Laren knew that Lug-Nut was in possession of sexy underwear and how Grady got into the picture are left unanswered.

Quotes
"I'm only gonna ask you this one time. Give me the Naughty Nightwear!"

Appearances
only appears in Fallout 3.

Bugs

 * On rare occasions, if you attack Lug Nut with a melee or unarmed weapon in V.A.T.S., a glitch will occur and you'll fall through the floor.
 * Lug-Nut can spawn with any Special Weapon or Power Armor.