Lizzie is a young girl growing up in Ballylaggin who receives a diagnosis of bipolar disorder at an early age. Her severe mood swings and periods of mutism frequently isolate her from her family and peers. Despite these medical challenges, she fiercely protects those she considers true friends and finds deep comfort in the unconditional acceptance offered by Hugh Biggs.

Hugh is a quiet, highly intelligent boy who excels academically while also playing rugby. He operates as a steadfast and fiercely loyal friend, taking immense pride in his role as a protector to those he cares about. When he meets Lizzie, he is immediately drawn to her unique personality and dedicates significant time to reading books at the library to better understand her mental health diagnosis.

Mark presents a charming front to the adults around him, securing their trust and regular access to their homes. Beneath this carefully constructed facade, he operates as a dangerous and highly manipulative predator who preys on the vulnerability of the children in his orbit.

Gibsie is a carefree boy who projects a jovial, lighthearted demeanor to the world. His home life becomes increasingly complicated by his parents' separation, leading him to rely heavily on the Biggs family for a sense of stability, warmth, and normalcy.

Claire is Hugh's younger sister and one of Lizzie's earliest friends. She is a cheerful presence who seamlessly integrates Lizzie and Gibsie into her family's daily routine. While she does not completely understand the darker aspects of her friends' internal struggles, she provides steady, unconditional companionship.

Caoimhe is Lizzie's older sister. As a teenager, she frequently experiences frustration over the disruptions Lizzie's mental illness causes in their household and her own social life. She oscillates between resenting her younger sibling and trying to protect her, while becoming increasingly entangled in her relationship with Mark.

Catherine is Lizzie and Caoimhe's mother. She attempts to shield Lizzie from the harshest criticisms of her mental illness, often insisting that her daughter is simply misunderstood. Her ability to care for her family is severely limited by her own recurring battles with cancer, which requires frequent hospital stays.

Mike is Lizzie's father. He frequently clashes with his wife over how to handle Lizzie's behavioral issues, favoring strict medical intervention over Catherine's more lenient approach. He is often impatient with Lizzie's symptoms and struggles to maintain peace in his household.

Peter is Hugh and Claire's father. Initially presented as a proud and engaged parent, he struggles with profound grief following a traumatic accident, leading to an emotional withdrawal that heavily impacts his relationship with his son.

Sinead is Hugh and Claire's mother, who works long hours as a nurse. She is a pragmatic and deeply caring woman who often serves as a surrogate maternal figure for the neighborhood children during their times of crisis.

Joe is Gibsie's father and Peter's best friend. He is an active and loving presence in Gibsie's life, regularly taking him swimming. His separation from his wife deeply upsets his son, though Joe attempts to remain connected to his children.

Sadhbh is Gibsie's mother. Following her separation from her husband, she quickly enters a new relationship and prepares for marriage. This rapid transition causes significant friction with Gibsie, who resents her new partner.

Keith is Mark's father and Sadhbh's new partner. His integration into the Gibson household creates extreme tension, finalizing the fracture of Gibsie's original family unit.

Feely is a close friend of Hugh and Gibsie who lives on a large farm. Despite his father's vocal disapproval of his vegetarianism and lack of interest in farming, Feely remains an integral, supportive member of Hugh's inner circle.

Shannon is one of Lizzie's few childhood friends at school. Coming from a difficult home environment, she is often targeted by bullies, prompting Lizzie to fiercely and physically defend her.

Joey is Shannon's older brother. He is highly protective of his sister and recognizes Lizzie's shared trauma, offering Lizzie quiet support and solidarity when he finds her crying alone at school.

Pierce is Hugh's rugby teammate. He is a social teenager who becomes a severe complication in Hugh and Lizzie's relationship dynamics as they grow older and begin attending parties.

Katie is a red-haired girl at Hugh's school. Reeling from a recent breakup of her own, she slowly begins to develop a connection with Hugh based on mutual friendship and understanding.