William "Bill" Denbrough is the main protagonist of Stephen King's "It". He is the leader of the Losers Club, ever since he lost his then-six-year-old brother George to the title adversary. He blamed himself for it, and his parents blamed him too. He became friends with the other members, who became his followers.
In the film, he is played by Richard Thomas as an adult and the late Jonathan Brandis as a child.