For the terminology, see sire.
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Sire is a character in Vampire: The Masquerade – Bloodlines. They are the sire of the fledgling. Their gender will be the opposite of the player if the player did not choose the homosexual background, and their clan is the same as the player.
Sire's history and personality is kept purposefully vague during the course of the game. They are rarely referenced by other characters and the player is not given the opportunity to ask about them, leaving their background to be pieced together and guessed at.
Events of Bloodlines
It is unknown when and how the sire found themselves in Los Angeles, where the game takes place.
In the original manual for Vampire: The Masquerade – Bloodlines, the story mentions that the fledgling apparently disappeared from a "seedy club" around the Sunset Strip in Hollywood. After picking up the fledgling from there, the two went to a Santa Monica hotel room together, which can be seen during the opening cutscene. There, the sire Embraced the fledgling, and the two were staked and brought in by the Camarilla shortly after.
During the opening cutscene of the game, Sebastian LaCroix refers to the sire as "a previously upstanding member of our organization", although it's not obvious if he is referring to the Camarilla specifically, or Kindred as a whole. If the player is Tremere and befriends Maximillian Strauss, he will compliment them on "transcending the fate woven by your anarch sire", suggesting that the sire was a part of the anarch movement or just anarch sympathetic.
As the player has a blood pool of 15 points, making them 8th generation, the sire would have had to have been 7th generation. That would make them a potentially powerful vampire, but not necessarily an old one.