The Grand Inquisitor (played by Rupert Friend) is the leader of all the Inquisitors, as his name implies. He's first seen in the first season of the animated series "Star Wars Rebels," which takes place a few years after "Obi-Wan Kenobi" is set. There, he pursues Kanan Jarrus and Ezra Bridger, seeking to defeat the would-be Jedi.
His appearance in "Rebels" indicates that his apparent fate in "Obi-Wan Kenobi" is not a permanent ending. In later seasons of "Rebels," it's also revealed that he had been a disillusioned Jedi and former guard of the Jedi Temple before falling to the Dark Side.