Dungeonmaster's Girlfriend

Term used in roleplaying game circles for a player - often literally the DM's girlfriend, sometimes a younger family member - who is getting special treatment from the gamemaster due to an out-of game personal relationship, specifically because the GM either wants to curry favor with them or is afraid of incurring their ire. This is often at the expense of other players, who showed up at the table to participate equally, but have somehow ended up playing sidekicks to the Dungeon Master's Girlfriend. (Or, if they're playing something other than Dungeons & Dragons, the Game Master's Girlfriend.)

This trope is much older than tabletop gaming and far from limited to gaming situations, though. Before there were RPGs, it was was a situation that would arise in theatre, film, and TV — the director would give his girlfriend a large or important role, or a producer would insist that his current girlfriend be given a part as a condition for his backing. In most cases, she is not competent enough to handle the role; in some, she's not competent at all.

This is not a trope about someone that does something only because their significant other is doing it.

In case of roleplay, the girlfriend or sibling might actually have an advantage by knowing the DM better than the other players without any favouritism coming into play.

Examples of Dungeonmaster's Girlfriend include:

Comic Books

Knights of the Dinner Table: The queen of this trope would be Heidi Jackson. She doesn't merely manipulate a local GM boyfriend, she actually acquires control of the Hackmaster RPG intellectual property (their world's Dungeons & Dragons) as the creator's heir's guardian and after a couple of game sessions decided to radically overhaul the game system in ways that led to rioting in the gaming community.
- By contrast Sara, the Gamemaster's cousin, subverts this trope so much that it's almost an inversion. B.A. cuts her even less slack than he does the other players, but he's constantly accused of giving her 'cousin favoritism' if she tastes success that the other players don't get. Especially if the reason why she was the only one to get anywhere because of her Genre Savviness, her niceness to in-game characters, or not being insane.

Film

Bullets Over Broadway. In this case, a mobster's girlfriend.
Underworld has Kate Beckinsale, who met the director on set and is now married to him. Guess who all the action in four films centers on.
In a very similar vein, Milla Jovovich in the Resident Evil film series met the director on set and they are now married. Might explain some of her telekinesis, super strength, replication and Healing Factor when the video games are mostly about Badass Normal people fighting zombies and mutant creatures.

Literature

One very early Dragon magazine short story (by Roger Moore... no, not him) used this trope in-character, when a tavernful of male adventurers were bragging of their power, achievements, and connections to still-greater beings. A woman at a private table kept snickering at their boasts, until they got fed up and confronted her with sexist remarks. She sneered at their petty claims, threw back her cloak to expose her magnificent armor, weapons, and other bling, and proclaimed: "My husband is the Dungeon Master." The boasters all looked up, anticipating a vengeful bolt from the blue, then very quickly evacuated the tavern when they realized no such retribution was coming.

Tabletop Games

Bride of Portable Hole (joke sourcebook for d20 System from EN Publishing) has a template for this.

GM’s Significant Other is a template that can be added to any intelligent living creature in a romantic laison with the Game Master. (hereafter referred to as the "base creature").

Built into the rules of the D20 version of Munchkin is a feat called "Shagging the DM". It has only one prerequisite, and it's in the title. It allows you to reroll one roll every 30 minutes. (The feat's description also points out that if you qualify for it, you probably don't need it.)

Web Comics

A case of Dungeon Master's Boyfriend in this arc of Something*Positive when PeeJee's boyfriend joins a game and just proves himself a Jerkass.
- Subverted in another arc where Davan plays DM straight, even though this causes Vanessa's character to die. At first it seems like she may be ticked/upset about this, but she isn't, she just ate too much junk food.
The Princess in Rusty and Co. claims to have this "VERY obscure and VERY powerful prestige class", which may explain her ability to save herself.
Throughout Heroes of Lesser Earth, the character known as Freya is given special treatment due to being the Dungeon Master's Girlfriend, most notably in the comic entitled Preferential Treatment.
Gabe of Penny Arcade gives this treatment to Vin Diesel, not because of a relationship, but because he is abjectly terrified of Vin Diesel.

Gabe: The skeleton lashes out at you, Vin, for... [Vin glares] ...zero damage! He missed! Again. And then... he died. So you get a million XP. And all the gold. And you win the whole game, forever.

It seems the DM's younger sister gets away with a lot more silly behaviour in Darths and Droids than the rest of the cast. For good or for better, she comes up with most of Star Wars's unusual fauna.
In this instalment of Sequential Art, the DM's girlfriend's character is stuck in a tunnel. "You'll get to roll again each time it's your turn. You'll wriggle free, I'm sure." She rolls a 1. In response, the DM has the next strip take place. "I found treasure?"

Web Original

Variation with Asenath Summerisle in The Binder of Shame; she's introduced as Deviant Boy's girlfriend while he's running a game. She herself is an okay gamer and ends up getting along fine with the guys, but he warps the entire game to revolve around her Mary Sue character; "She got to play hero while the rest of us swam through raw sewage being chased by vaguely alien sex slavers."
Things Mr. Welch Is No Longer Allowed to Do In An RPG touches the subject repeatedly:

421. I will not convince the GM's noob GF to play a psychotic combat monster.
435. No bribing the DM's new GF with chocolate so he'll go easy on us.
1347. Hooking up with the gamer chick the Storyteller was angling for gets me banned from the game.
1905. No nailing the GM's girlfriend's character.

- In another project of TheGlen, analysis of The Lord of the Rings from PoV of a millennial gamer, he makes a case of Frodo being a DMGF character:

As the realization set in that the entire Fellowship of the Ring was just an extremely badly run game, Frodo has gone from the DM's pet character to be revealed what he actually was. The poor bastard being played by the chick the DM had eyes for. I was fooled into thinking Frodo was some sort of Pollyanna Mary Sue type character, but the more I watched the more I felt sorry for the poor girl playing the hobbit. It rapidly became obvious Frodo didn't want to be there anymore.