Galileo Galilei
Galileo is the protagonist of the play, who strives to prove a heliocentric system of astronomy. This theory claims that the Earth, and the other planets, revolve around the sun. At the time, the commonly held belief, upheld by the government as well as the Christian Bible, was that everything, including the sun and the moon, revolved around the Earth as the center of the universe. Galileo tries to prove his hypothesis, despite opposition from the Church and the Inquisition. He also takes the idea of the telescope, which Ludovico makes him aware of, and presents it to the Republic of Venice.
Galileo eventually recants everything he has ever said, which disappoints and angers his dedicated and loyal followers, choosing life over martyrdom. However, when Andrea comes to visit, he reveals that he has secretly been writing his theories in his Discorsi, which proves to his protege that he has always put science first. Galileo puts his trust in reason, and tirelessly works on behalf of finding the truth. He believes in the importance of doubt over faith, the scientific impulse to try and understand the world using a critical eye. He is also a man of pleasure, enjoying the finer things in life, like a nice apartment and fine wines.
Andrea Sarti
Andrea is Galileo's student, a young boy at the beginning of the play, and the son of Galileo's housekeeper. He follows Galileo throughout the play and is a dedicated supporter of his work, although he sometimes requires more complicated theories to be explained further to him. Andrea cannot understand why Galileo initially tells him not to share their ideas or hypotheses. He is passionate about proving Galileo's hypothesis and is confident in his teacher Galileo. He does not believe that Galileo will recant his ideas when brought before the Inquisition, and is particularly devastated when he does. He goes on to work in the scientific sector, despite it being controlled by the church and the Inquisition. Andrea takes Galileo's teachings to heart and carries forward his teacher's ideas across the Italian border and into the future.
Mrs. Sarti
Mrs Sarti is Andrea's mother and Galileo's housekeeper. She represents the everyday person of the time and is hesitant towards Galileo's ideas, as they conflict with the church's teachings. She is opposed to Galileo teaching her son such ideas, as they go against the church. In the opening scene, she is more concerned with paying the milkman than taking in Galileo's ideas, but she is intent on providing for Galileo, and remains loyal to him, even in the midst of the plague.
Ludovico Marsili
Ludvico is a young nobleman, and seeks Galileo's tutelage, when his mother tells him to find himself a teacher. He questions Galileo and his theories, but also gives him information about the telescope, which Galileo uses for his own gain. Later, Ludovico becomes engaged to Virginia, Galileo's daughter, but he calls off the marriage because of the controversy surrounding Galileo's work, and Galileo's unwillingness to turn away from it.
Virginia
Virginia is Galileo's daughter. She is portrayed as loyal and loving, but somewhat simple-minded, never taking to Galileo's ideas and believing more in her religion than in science. She prays that Galileo will recant towards the end of the play, and is pleased when he accepts religion and withdraws from his scholarly pursuits.
Sagredo
A friend of Galileo's, who questions him about the place of God in his hypotheses.
The Little Monk
A member of the Church, who, based on his position, ought to oppose Galileo's ideas and remain loyal to the church, but who becomes interested in Galileo's theories and becomes his pupil.
