Although the priest Ralph de Bricassart appears on fewer pages in The Thorn Birds than the main character Meggie, his role has sparked the most discussion. Exceptionally handsome, with some critics drawing comparisons to Rhett Butler, Ralph never exploits his good looks to indulge in the numerous sexual opportunities presented to him, apart from a brief indiscretion with Meggie. However, his ambition drives him to scheme and inherit a fortune that would have otherwise gone to Meggie's family, the Clearys. Ralph is a man destined for power, and he wields it effectively, often for the benefit of others. In 1943, as a bishop in the Curia in Rome, he stands up to German General Kesselring and soon after shows compassion to a frightened, confused, yet intelligent young German soldier trapped in St. Peter's Basilica after coming there to pray.

The three Cleary women include Fiona (mother of Meggie and seven sons), who is disillusioned and exhausted from betrayal and overwork; Meggie, who seems to lose her love twice—first when Ralph leaves her to stay in the priesthood, and then when her son by Ralph joins the priesthood and dies young in a drowning accident; and Meggie's daughter, the willful and oddly unattractive Justine, who becomes an actress and seeks the liberation and freedom her mother and grandmother never experienced.

Fiona, also known as Fee, loves her children, especially her eldest son, fiercely and unconditionally but struggles to express it naturally due to the hardships of her life. Her love is evident at the beginning of the novel when, despite being very poor, she buys Meggie a doll for her birthday.

Meggie, the novel's central character, is torn by love and fights against her feelings for Ralph while striving for a better life for herself and her family. Her unwavering dedication to Drogheda, her ranch, sustains her through numerous challenges. When her son Dane dies, Meggie uses the fact that Dane is also Ralph's son to persuade Ralph to use his influence to bring Dane home for burial.

Fiona's husband, Meggie's father, Paddy Cleary, is a hardworking, simple, good-hearted, and perpetually bewildered man who dies early in the story. A firm believer in traditional values and staying in one's place, Paddy cannot afford to keep his children in school and needs them to work on the farm. He also upholds strict distinctions between women's and men's work, policies that are hard on Fee and their eldest son, Frank, creating a rift between father and son. Despite these challenges, Paddy instills in his children a strong work ethic and a love for the land. An Irishman who fled to New Zealand after killing an Englishman, he sets the stage for the novel's beginning.

Paddy's sister, Mary Carson, is a wealthy and cruel woman who harbors an obsession with Ralph de Bricassart. She decides to bequeath the majority of her substantial fortune to him and the church. After hosting a lavish celebration for her seventy-second birthday, she declares that she will die that night, and she does.