Magos is a grieving mother who cuts open her deceased son's chest to extract a piece of his lung, keeping it in a jar. She returns to her childhood home in Mexico City, where she begins feeding the lung chicken broth and raw meat. As the lung grows into a creature, she treats it with a fierce, unwavering maternal protectiveness, viewing it as a continuation of her role as a mother.

Lena is a gifted surgeon and Magos's best friend from university. She grew up with an abusive mother who convinced her she was a demon, leaving her with a lasting belief that she is unworthy of conventional love. To fulfill her need for physical comfort and maternal care without romantic vulnerability, she routinely hires sex workers to bathe her and dress her in pajamas.

Joseph is Santiago's father, a man whose profound grief leaves him initially bedridden and unable to function. He expresses his sorrow openly, crying and physically wasting away, which contrasts sharply with his wife's silent mourning. He eventually moves to Williamsburg, New York, where he attempts to construct a new, functional life while hiding his complicated family history.

M begins existence as a carnivorous creature grown from a piece of Santiago's lung, initially possessing an arm-tail and a wide mouth of sharp fangs. He relies entirely on Magos, Joseph, and Lena for food and shelter. As he develops a human-like body, he experiences a constant, intense physical hunger and struggles to understand the societal expectations his caretakers place upon him.

Santiago is the 11-year-old son of Magos and Joseph, born with an underdeveloped single lung that ultimately claims his life. During his brief life, he vacillated between active play and severe respiratory distress, frequently drawing monsters under the dogwood tree in his yard. His death serves as the catalyst for the events that tear his family apart and create M.

Uncle Luke is Joseph's closest living relative, an elderly man who lives in the vestiges of his family's former wealth. He does not speak, relying instead on grunts and a notepad to communicate. Decades of absorbing family tragedies have left his body physically bent and gnarled, giving him a profound understanding of loss.

Peter is Joseph's fiancé, living with him in a Williamsburg apartment. He loves Joseph and desires to be fully integrated into his family life. However, he frequently finds himself confused by the strange dynamics between Joseph, Magos, and M, unaware of the massive secrets his partner hides regarding M's origins.

Lucía is Magos's mother, living on a retirement pension in her family home in Mexico City. She insists on confronting loss directly, organizing a memorial mass for Santiago against her daughter's wishes. She attempts to guide Magos back toward a functional relationship with Joseph while managing the daily upkeep of her household.

Jackie has worked as Lucía's housekeeper for twenty years. She possesses a deep knowledge of local folklore and superstitious tales, quickly warning Magos against feeding the dead flesh she finds in the bedroom. She prioritizes the safety of the household and its pets over Magos's unnatural experiment.

Thomas is a young man who works at a Berlin art gallery and becomes romantically involved with M. He introduces M to the complexities of human physical intimacy and attempts to understand his unique physical boundaries and sudden urges.

Sam is a man living in New York who connects with M through a dating app. He holds eccentric, extreme beliefs about being consumed by a deity to reach another plane of existence, actively seeking out someone who can fulfill this highly specific desire.

Romy is an NYU student who works at a used bookstore in New York. She befriends M, viewing his unusual behavior as interesting rather than alarming, and actively encourages him to participate in social activities like parties and digital dating.

Carmina is a sex worker operating in Mexico City. Lena hires her for a strictly non-sexual service, paying her to provide thorough physical washing and the comforting routine of being dressed in clean pajamas.