Twenty-year-old Will Hunting of South Boston is a self-taught math genius who was recently paroled after completing a prison term. He works as a janitor at MIT and spends his free time drinking with his friends Chuckie, Billy and Morgan. He had a very traumatic past and due to that he has multiple mental health disorders, like BPD and is afraid of showing himself to others. Throughout the movie he is unwilling to fulfil his potential and better himself.

Dr. Sean Maguire is the deuteragonist of the 1997 drama film Good Will Hunting. He is the court-ordered therapist of Will Hunting, a troubled math genius and orphan, who tries to break through the young man's defenses and help him overcome his childhood trauma. He helps him become aware that no matter how hard he is trying to live small, there is a part of him that wants to achieve his full potential. Slowly, Sean begins to earn Will's trust. Sean gently steers Will to find his inner resources, face his fears, and take the risk of emotional intimacy.  

He is the best friend of Will Hunting, despite Will having high standards for others in order to make him trust people. He is also loyal and charming to Will, although just not in the same intellectual level. In Chuckie's mind, Will's life is an insult to all the men who work construction and wish they could be doing something else. Chuckie makes this clear when he says that the best part of his day is going to pick Will up in the morning and hoping he won't be there. 

Skylar is Will's girlfriend who he meets in a bar one night. She grew up in England, she comes from money and goes to Harvard. Skylar appreciates Will’s combination of intelligence, candor, and sincerity– it’s refreshing after the last four years she’s spent with typical, pretentious Harvard students. She has to break down Will’s defenses. Despite the fact that Will does his best to keep Skylar at arm’s length, she falls in love with him quickly. Now, if only she could figure out how to convince him to admit he loves her, too.   

Gerald Lambeau is a math professor at MIT and winner of the Fields Medal, generally regarded as "one of the highest honors a mathematician can receive."