Jack Powell is the titular protagonist of Jack.
He was portrayed by the late Robin Williams, who also portrayed Mork, Popeye, John Keating, Peter Banning, Daniel Hillard, Alan Parrish, Phillip Brainard, Lance Clayton, Genie, Batty Koda, Ramón, and Lovelace.
Jack is first seen as a newborn by his parents Brian and Karen.
Ten years later, he is now a child stuck in an adult's body. At first, a group of four boys feel he cannot attend school. He scares them away by dipping a fake eye into slime and throwing it at them through the window. His tutor, Lawrence Woodruff suggests that he should attend school. His parents had agreed at first. On his first day of school, he is exploited by the other kids to win at a basketball game against the school bullies. He also got adult magazines and other items. At first, he had to be normal, but begins a relationship with his teacher, Miss Marquez, but goes through a fall and is taken to the school nurse. Because of his Werner syndrome and the dangers of his health, his parents decide to withdraw him from school, much to his dismay. Jack sneaks out of the house and goes to a bar where he ends up being drunk and meets a man named Paulie, but he gets into a fight with a bully and the two get arrested. After being released from jail, he decides to lock himself in his room and decides to stay in there after realizing that the outside world is too dangerous for Jack. Karen becomes concerned about Jack. However, his friends show up at his house and asks if Jack can come out and play. Eventually, Louis brings all of his friends as they shout out "Can Jack come out and play?" and Jack finally leaves his room and begins to do the fun activities. The next day, he finally returns to school and continues the rest of elementary school. Seven years later, Jack is now an old man and is at his high school graduation as class valedictorian and delivers his speech to his friends and his parents. At the end of the film, he is last seen driving away with his friends as they begin their future together.
- Originally, Tom Hanks and John Schneider auditioned for the role of Jack Powell, but they turned down the role before the role was given to Robin Williams.