The Tyrant that pursues Jill Valentine in the original Resident Evil 3: Nemesis is called Nemesis.
Nemesis is a bio-organic weapon created by the Umbrella Corporation. Its mission is to eliminate surviving members of S.T.A.R.S., the police special unit that uncovered Umbrella’s experiments in the original Resident Evil. Jill Valentine becomes its principal target during the Raccoon City outbreak.
Unlike many earlier Tyrants, Nemesis can use weapons, speak limited words, and pursue its target persistently across the city. The character’s distinctive appearance includes a black coat, exposed teeth, and extensive surgical modifications.
Nemesis is sometimes confused with Mr. X, another Tyrant from Resident Evil 2. Mr. X stalks Claire Redfield and Leon S. Kennedy, while Nemesis specifically serves as the principal pursuer in Resident Evil 3. The subtitle “Nemesis” refers to this bio-weapon rather than to Jill or Umbrella.