The character of Galileo Galilei is based on the historical figure by the same name (1564–1642). Brecht's Galileo is characterized from the beginning of the play as a brilliant scientist who searches for truth while remaining self-interested and focused on maintaining his quality of life by selling his designs and flattering potential patrons. Galileo is frustrated and even frightened by the church's censure of the Copernican-leaning aspects of his theories. Twice in the story, Galileo publicly acquiesces to the mandates of the church while privately continuing his research. Galileo and his assistants view his recantation as weak and as a betrayal of science, while his continued research offers a small redemption.
Virginia Galileo, the daughter of Galileo Galilei, is loyal and caring to her father, regardless of the many ways that his actions negatively affect her throughout the play. Virginia is also loyal and faithful to the Catholic church, no matter how her father's theories challenge its principles. Virginia becomes engaged to Ludovico and looks forward to her impending nuptials. She has an emotional attachment to Ludovico and is disappointed by their broken engagement, which results in her continued role as a companion and caretaker for her father as he reaches old age.
In the beginning of the play, Andrea Sarti is a small boy, the son of Mrs. Sarti, Galileo's housekeeper, whom Galileo has taken on as a kind of student and errand boy. As he grows up, Andrea takes on the role of an assistant and learns astronomy from Galileo. He is shocked by Galileo's recantation, and he leaves Galileo to pursue his studies elsewhere. Toward the end of the play, he returns to visit Galileo, who is under house arrest. Andrea regains his faith in Galileo when he discovers that the astronomer has finished another treatise in secret. Andrea is given the chance to carry on Galileo's legacy when he takes the book to another country for publication. He becomes the scientist who searches for truth and encourages others to do the same with their own observations. In stagings of the play, two actors (a young boy and a man) portray Andrea Sarti's character at different ages.
Sagredo observes astronomical phenomena with Galileo, serving as an assistant in his scientific research, and comprehends their implications for the Catholic church. Sagredo asks Galileo whether he should proceed with his findings, knowing that the church will not receive them well. Sagredo remains a faithful assistant to Galileo throughout his research, until Galileo recants his teachings.
Federzoni becomes acquainted with Galileo because he is a lens grinder for the telescopes that Galileo designs. He eventually becomes an assistant in Galileo's research. Federzoni participates in the research with Galileo and the other assistants, but he is not educated in the university manner. His inability to speak Latin (and his ability to comprehend astronomy) is one of the reasons that Galileo chooses to write in the common language. Federzoni is shocked and disappointed by Galileo's recantation and feels betrayed by the astronomer's inability to defend his research.
Fulganzio, the Little Monk
Fulganzio, the Little Monk, is a clergyman and the son of peasants, and he is fascinated by Galileo's astronomical research. He assists Galileo on some of his projects. Even though he has observed the same phenomena as Galileo and agrees with his findings, the Little Monk argues that the decree against Copernican cosmology is beneficial for the common people, whose lives would lose meaning without religion.
Ludovico Marsili is a young member of a noble Italian family. His mother wishes him to broaden his education by studying science with Galileo, so Ludovico engages Galileo as a tutor. Ludovico is not interested in science and has little passion for it, but he maintains a relationship with Galileo and Virginia, with whom he eventually becomes engaged. Ludovico views the engagement as a mutually beneficial arrangement, which allows him to break the engagement when a relationship with Galileo becomes disadvantageous. Ludovico, as a nobleman loyal to the church, disapproves of Galileo's teachings because of their challenges to church teachings about the Bible as well as to the social order.
Cardinal Barberini (Pope Urban VIII)
Cardinal Barberini (1568–1644) is one of the two cardinals who is tasked with delivering the pope's decree against Copernican cosmology to Galileo. Cardinal Barberini sparks a friendly scientific debate with Galileo before Cardinal Bellarmin communicates the warning to Galileo. Cardinal Barberini trained as a mathematician and is interested in science. He supports Galileo's research, even in the first years after he is elected Pope Urban VIII. After Galileo's 1632 publication of a treatise that promotes Copernican cosmology, however, the now Pope Urban VIII acquiesces to the demands of his office. The pope reluctantly allows the Inquisitor, who is responsible for investigating heresies, to interrogate Galileo and convince him to recant his theories.