The Familiar is an unusual role in that it is not assigned at the start of the game, but recruited by a Vampire. During the night, the Vampire may choose (by targetting themselves first) to change their night action to turn a player into a Familiar, instead of killing as normal. Once recruited, the Familiar is permenantly aligned with the Vampire and shares their win condition.
The Familiar is granted night time immunity on the night that the Vampire recruits them. This includes one off attacks such as the Militia or the Witch potion.
The Familiar has the same powers as the village-aligned Stalker and can talk with the Vampire in secret at night.
If the Vampire is killed and their Familiar is still alive, there is a chance that the Familiar will be able to revive their master. On the Vampire's death, a living player will be marked. If that player is killed for any reason, then the Vampire will be reborn in the Familiar's body.