Hester Swain: Swain is a forty-year-old “tinker” woman who has lived on the Bog of Cats her entire life. When she was seven years old, her mother, Josie Swane (not pictured in the play), abandoned her on the Bog. She has been waiting there for her mother ever since. Hester has a daughter, Josie, with Carthage Kilbride. She is very resentful that Carthage has left her to marry Caroline Cassidy, the daughter of wealthy landowner Xavier Cassidy so that he can inherit the Cassidy farm. Before the action of the play, Hester, with the help of Carthage, murdered her half-brother Joseph Swane, out of jealousy for his life with their mother.
Carthage Kilbride: Carthage is Hester's former lover with whom she shares a daughter, Josie Kilbride. He leaves Hester due to his greed for land and social ambitions. At thirty years old, he is ten years Hester's junior. He began a relationship with her at sixteen years old. During their relationship, Hester encouraged Carthage to have ambitions beyond his social class as a labourer's son, even giving Carthage the money to buy his first land. During By the Bog of Cats, the conflict between Hester and Carthage centres around his attempts to force Hester to leave her home on the bog and give him sole custody of their daughter.
Josie Kilbride: Josie is Hester's and Carthage's seven-year-old daughter. She is the same age that Hester was when her mother left her and the Bog of Cats. She is a generous, loving little girl who enjoys spending time with both her mother, father and mean-spirited, greedy grandmother. Josie sings several songs throughout the course of the play which foreshadow her death on the Bog of Cats.
Mrs Kilbride: She is Carthage's mother and in her sixties. She looks down on Hester because Hester belongs to the “tinker” class and tries to hide the fact that her own grandfather was a tinker. Mrs Kilbride calls her granddaughter a “little bastard” because she was born out of wedlock. She is very greedy and constantly focuses on issues of social class and money. She even cheats her granddaughter for money while they are playing cards. Mrs Kilbride also has an inappropriate attachment to her son, Carthage. She appears in a white dress at Carthage's wedding and interrupts the bride's father's speech to make her own toast dedicated to her son.
Monica Murray: She is Hester Swane's sixty-year-old neighbour on the Bog of Cats. While she does at times seem sympathetic to Hester, she attempts to carry out the community's wishes and convince Hester to leave her house on the bog so that Carthage and his new bride can move in there. She also tries to convince Hester to accept the marriage between Caroline and Carthage. Despite this, Hester begs Monica to tell her stories about her mother.
The Catwoman: The Catwoman is a strange mystical figure who most closely echoes the blind prophet Tiresias from Greek myths. She eats mice and drinks milk from a saucer. She has magical powers that include prophecy and talking to ghosts. It is hinted that she may have some kind of intimate relationship with the eighty-year-old priest, Father Willow. Even though the community sees the Catwoman as eccentric, they still invite her to all of their functions and consider it bad luck not to do so.
Xavier Cassidy: Xavier is a farmer who schemes to marry his daughter to someone he can easily control since his daughter cannot inherit his land. He convinces Carthage to marry Caroline and promises to sign over the farm to him. Hester believes that Xavier was in love with her mother and attempted to woo her before she left the Bog.
Caroline Cassidy: Caroline is the twenty-year-old daughter of Xavier Cassidy who marries Carthage Kilbride. To add insult to injury, Hester often babysat Caroline as a child. Caroline is very meek and follows orders from both Xavier and Carthage. However, she does express guilt about the marriage to Xavier because she feels that they are being unfair to Hester.
The Ghost Fancier: This ghostly figure appears at the beginning of the play to collect the spirit of Hester. However, Hester is still alive, so the Ghost Fancier realizes he is too early. He comes back at the end of the play after Hester kills Josie, and Hester kills herself.
The Ghost of Joseph Swane: Hester's brother's ghost returns to the Bog of Cats to find out why Hester killed him. He believes that Hester was just trying to steal money from him. However, in their conversation, Hester reveals that she was jealous of his relationship with their mother.
Father Willow: He is the town priest who officiates Caroline and Carthage's wedding. He is inappropriately close with the Catwoman. He appears confused and forgetful.
Young Dunne: Often referred to as the waiter, who serves the guests at the wedding of Carthage and Caroline.