Lady Lyanna Mormont was the young Lady of Bear Island and thus the head of House Mormont of Bear Island ever since the death of her mother, Maege Mormont. She was the niece of Lord Commander Jeor Mormont of the Night's Watch and the first cousin of Ser Jorah Mormont. She pledged House Mormont's forces to House Stark upon meeting Jon Snow, Sansa Stark, and Davos Seaworth, making her house one of the few loyalists to fight for the Starks against House Bolton at the Battle of the Bastards. Afterward, she was the first to declare Jon Snow the King in the North during a gathering of the Northern lords at Winterfell.

Actor must be able to do a heavy Northern Yorkshire English accent and also sound like a 12-year-old child. Despite her young age, Lyanna is shown to be a competent, effective, and intelligent leader, in sharp contrast to the psychopathic and sadistic Joffrey, timid and passive Tommen, lazy and careless Robert Baratheon, arrogant and heartless Cersei, and developmentally challenged Robin Arryn. While she emanates self-confidence, she does not hesitate to consult her advisors if she sees the need. At the same time, she is not overly-reliant on her advisors, and will wave aside her maester if she has reached a firm decision already. As a female and the only living heir of a major Northern house, Lyanna challenges and strives to live up to the expectations placed upon her, and behaves in a stern, no-nonsense manner when negotiating with other houses. She is extremely proud of her house, and she is devoted to protecting Bear Island's people, indicating that she views this responsibility with the utmost solemnity.