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. Afterwards, she was the first to declare Jon Snow the King in the North during a gathering of the Northern lords at Winterfell. She later took part in the Battle of Winterfell, during which she was killed by an undead giant, whom she managed to destroy before succumbing to her wounds.
Lyanna Mormont was the daughter of Maege Mormont. She was named after Lyanna Stark, who died years before she was born. After her mother’s older brother Jeor joined the Night’s Watch and his only son Jorah exiled to Essos, Maege Mormont became Lady of Bear Island. When Maege left to fight in the War of the Five Kings, Lyanna ruled Bear Island in her mother’s stead, eventually taking Maege’s place upon her death in combat.
House Mormont’s domains are poor on their heavily forested island, vulnerable to attack from the sea, meaning that the Mormonts have had to develop a strong tradition of warrior women to defend their homes from surprise raids. Maege Mormont personally led and fought beside the Mormont contingent in Robb Stark’s army. Maege’s daughter Lyanna was similarly raised, expecting to be a ruler and commander – unlike many other Houses, even in the North, that usually do not allow women to rule.
Along with the other Stark vassals of the North, House Mormont’s army was slaughtered at the Red Wedding, leaving their home garrison with only sixty two remaining warriors – led by a ten year old Lyanna, because most of their people of fighting age had been killed.
Stannis Baratheon has come to Castle Black and sent ravens to all the noble houses of the North with messages demanding them to join his cause. Lyanna Mormont sends back a rebuff that she and Bear Island know no other king but the King in the North, whose name is Stark. An irate Stannis shows the message to Jon Snow, who smiles briefly and points out that, much like the Free Folk, the Northmen are loyal to their own.
Lyanna is one of the Northern leaders being visited by Jon Snow, Sansa Stark, and Davos Seaworth, who are attempting to rally the North against House Bolton who currently occupies Winterfell. Sansa and Jon try to flatter her, but Lyanna brusquely brushes them off, demands to know why they are at Bear Island, and questions whether Jon and Sansa are technically even Starks, the former being a bastard, and the latter being twice married into enemy houses. She refuses to offer her men to them until Davos steps in.
He tells her that the real war is not between different houses, but between the living and the dead; a divided North cannot hope to stand against the Night King. After some reflection, Lady Mormont agrees to help, but can only offer sixty-two fighting men (as so many of their soldiers were massacred at the Red Wedding). Undaunted, she insists that the hardy warriors of Bear Island are worth many times their own number in battle. She personally leads these men when Jon travels to Stannis’s old camp.
Lyanna is present when Jon and Sansa parley with Ramsay Bolton ahead of their battle. She doesn’t speak, but scowls at Ramsay when he mockingly says he’ll pardon any Northern lords for treason if they lay down their arms (knowing everything he says is a lie). She does not fight during the battle itself, and presumably stayed behind at the camp.
Lyanna is among those present at Winterfell after the battle. As the knights of the Vale, the many Northern lords, and the Free Folk bicker, she stands in the assembly and calls out the Manderlys, Glovers, and Cerwyns for refusing Jon and Sansa’s call to arms. She then declares that she doesn’t care if Jon is a bastard, because Ned Stark’s blood runs through his veins. She is the first to proclaim Jon the new King in the North, and is quickly followed by the others.
Lady Lyanna is present at a meeting of Jon Snow’s allied forces. When Lord Robett Glover protests at the King in the North’s orders that everyone over the age of 10 be trained for combat, regardless of gender, Lyanna counters that she will train all on Bear Island against the coming White Walkers, stating that she does not intend to sit by her fire knitting while men fight to defend her. Lord Glover tries to apologise and withdraws his complaint.
When Jon is called to Dragon stone by Daenerys Targaryen regarding a potential alliance, Lyanna states that with winter here, the North needs their King with them. Jon responds that this is something he must do if they are to survive, so he departs with Davos while leaving Sansa in control of the North.
Lyanna Mormont is present at Winterfell when Jon Snow returns with Daenerys and her armies. When Jon introduces Daenerys to Sansa, Lyanna looks distrustfully to the Dragon Queen. Later at a meeting, Lyanna firmly criticises Jon Snow for renouncing his crown as King in the North, even going so far as openly questioning whatever Jon is after he declared Daenerys as his queen. However, Lady Mormont sits down when Jon states that he brought the North a strong ally and that it doesn’t matter whether he is their king.
As Winterfell prepares for the imminent arrival of the army of the dead, Lyanna and her cousin Jorah have an argument in the courtyard. Jorah strongly advises her to remain out of the fray, as she is the future of House Mormont, but Lyanna stoutly refuses, wishing her cousin good luck before leading her men away.
During the Battle of the Bastards, Lyanna commands the defence of Winterfell’s courtyard. After the castle’s walls are breached, a wight giant storms through the gates of Winterfell and knocks Lyanna out of his way. Injured, she gets up and charges toward the Giant, but he lifts her, crushing both her and her armour with his clenching fist. Near death, Lyanna musters the strength to deliver a final blow, a dragon glass dagger straight into the Giant’s eye, killing him. The giant falls dead and Lyanna succumbs to her injuries. When the Night King brings back the dead, Lyanna is resurrected, but once the Night King is killed by Arya Stark, she is wiped out along with the rest of the wights and White Walkers (unless she was already destroyed in the fighting).
Together with the others fallen in the battle, Lyanna’s body is placed on a funeral pyre. Jon Snow sadly looks upon her before he lights the pyre.
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. Though Lyanna shares her family’s fierce loyalty to House Stark, she was initially reluctant to help Jon and Sansa fight the Boltons until Ser Davos convinced her, comparing their recent rises to power and informing her of the greater threat beyond the Wall.