What is the name of the first Slayer?

The first Slayer is named Sineya.

In Buffy the Vampire Slayer mythology, Sineya was a young girl in prehistoric Africa who was transformed into the first Slayer. A group of ancient men known as the Shadow Men forced mystical power into her, combining her essence with that of a demon. The ritual gave her extraordinary strength, speed, endurance, and predatory instincts.

Sineya’s creation established the line of Slayers that follows her. Traditionally, only one Slayer is active at a time, and a new Slayer is called when the current Slayer dies. Buffy’s dreams and visions sometimes connect her with Sineya, who appears as a silent, primal figure rather than as a conventional mentor.

The name is easy to confuse with later Slayers. Buffy Summers is the central modern Slayer, Faith Lehane is a later Slayer activated after Kendra’s death, and Nikki Wood is another predecessor in the line. Sineya is not simply the first Slayer chronologically known to viewers; she is the origin of the Slayer tradition itself.