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 player character 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.

Fallout: New Vegas

 * One 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 fifty caps instead of 100.
 * One can flirt with Lady Jane to have her pay her debt to the Atomic Wrangler.
 * One can invoke some humorous dialog between you and The Lonesome Drifter; he informs you 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.
 * One can imply that Cass is flirting with you, to which she will say their relationship will not go any farther.
 * One can access some unique dialogue options, and eventually seduce 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.

Ranks

 * Rank 1: Women suffer +5% damage in combat, and are easier to persuade in dialogue.

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.