The Master

He arrives wearing someone else's face, a warm and helpful stranger who turns out to be anything but. The Doctor's oldest rival, a renegade Time Lord bent on wounding her personally, he talks fast, laughs harder, and delights in being the smartest, cruellest thing in any room he enters.

Sacha Dhawan plays him as pure live wire, swinging between wounded intimacy and gleeful cruelty within a single sentence. The writing gives him genuine history with the Doctor, real hurt beneath the theatrics, and Dhawan makes that hurt land even while he's grinning, dancing, and burning everything down around him.