Lady Killer

Lady Killer is a perk for male player characters in Fallout 3, Fallout: New Vegas and Fallout 4.

Fallout 3 and Fallout: New Vegas
In combat, the player character does +10% damage against opponents of the opposite sex. Outside of combat, one sometimes has access to unique dialogue options when dealing with the opposite sex, similar to the Sex Appeal trait. In Fallout: New Vegas, it is complemented by the Confirmed Bachelor perk.

Fallout 3

 * Nova in Megaton will provide the Lone Wanderer with the password to Moriarty's terminal.
 * Brianna in the Blood Ties quest will provide the player character with the password to Ian West's cell.
 * One can talk the AntAgonizer into stopping her attacks on the town and ending her feud with The Mechanist. The Lone Wanderer also gets her costume.
 * One can talk Fantasia into stealing Dukov's key for the quest You Gotta Shoot 'Em in the Head. It is possible to do this without the perk though, but Lady Killer eliminates the possibility of a Speech check failure.
 * One can talk Agatha into giving them the key to her ammunition box to help the Lone Wanderer during the quest Agatha's Song.
 * In Operation: Anchorage, one can calm Specialist Olin down and get more detailed information on questions. Also, the conversation will not end after each question.
 * In The Pitt, Vikia will prove to be immune to the perk and will mock women who are manipulated by it. She goes on to say that she wouldn't sleep with the player character, no matter how impressive he is.

Fallout: New Vegas

 * The Courier can flatter Old Lady Gibson into giving them the thrust control modules for free during the quest Come Fly With Me.
 * One can make Janet immediately fall in love with Jack during Young Hearts.
 * A hooker named Dazzle in the Gomorrah courtyard will have sex with the player character for 50 caps instead of 100.
 * One can flirt with Lady Jane to have her pay her debt to the Atomic Wrangler.
 * One can get Sweetie to hand over over the key to St. James' room in The Coyotes sidequest.
 * One can invoke some additional dialogue between the Courier and The Lonesome Drifter; he informs the Courier that he lived in Montana and his father left before he was born. The player character has the choice of worriedly asking "Montana? Oh god, you wouldn't happen to be seventeen would you?" to which he replies in the negative, and the player character responds by attempting to change the subject.
 * One can compliment a ghoul or human female prostitute on a lower Brimstone table in Gomorrah during How Little We Know in exchange for Cachino's location.
 * The Courier can imply that Cass is flirting with them, to which she will say their relationship will not go any further.
 * One can access some unique dialogue options, and eventually seduce Light Switch 01 and Light Switch 02 in Old World Blues.
 * In Old World Blues, one can talk to Dr. Dala about their breathing for the quest Coming Out of Her Shell.

Fallout 4
Just as before, Lady Killer offers bonus damage against women. However, it does not confer any additional speech options due to a refurbishment of the dialogue system, and instead makes them easier to persuade in dialogue or be pacified via the Intimidation perk.

Behind the scenes
The term "Lady Killer" is slang, dating back to the 19th century, referring to a man who is very popular and/or influential with women.

Bugs
In Fallout 4, due to several scripting typos, ranks 2 and 3 provide no actual benefit to pacification success rates with Intimidation.

Ladykiller Donjuán Tueur de Femmes Czarna Wdowa Женоубийца Жінковбивця Lady Killer