Lyanna Mormont
The child Lady of Bear Island, all of ten years old and fiercer than lords five times her age, who suffers no condescension whatsoever.
The fuller tale
Bear Island's answer to a decade of catastrophe was a ten-year-old who takes no prisoners conversationally. Lyanna Mormont — named for the Stark girl her lady mother rode beside in memory of — inherited her island when the war ate the family ahead of her, and rules it from a hall carved with the Mormont truth: a spear-carrying woman over the door, because Bear Island's men are often at sea and its women fight their own raiders. The tale first hears of her as a letter — a curt reply to a king's summons declaring that Bear Island knows no king but the King in the North, whose name is Stark — and first meets her enthroned between advisers three times her size, none of whom she appears to need.
Her great scene is the making of a king. When Jon and Sansa come recruiting with the Bolton war looming, the lords of the North hedge — and the smallest head of the smallest house rises to shame them by name, house by house, absence by absence. After the battle she does it again in the other direction: it is Lyanna Mormont who stands in Winterfell's hall, calls out every lord who stayed home, and declares for the White Wolf — the speech that crowns Jon Snow King in the North, delivered by a child too young for the feast's wine. Sixty-two men is Bear Island's whole strength, she notes, and every one fights like ten mainlanders; the tale never once suggests the exchange rate is padded.
She became, in three scenes, the show's emblem of the North itself — unimpressed by titles, immune to condescension, keeping faith by her own lights and no one else's. Grown lords rearrange their arguments when she frowns; a queen's courtesies get audited in real time. The tale's affection for her is the affection the whole story holds for the North's stubbornness, concentrated into its smallest and least negotiable container.
The page and the screen
In the books, Lyanna Mormont is a single glorious sentence: the ten-year-old whose letter about the King in the North, whose name is STARK, is read aloud in Stannis's camp — and everything else about her is the show, which took the letter, gave it a face, and built a fan-favorite from the extrapolation. Book Lyanna is alive on Bear Island, youngest of five Mormont sisters in a succession the books track carefully; the show simplified the family to put the island directly in her small hands. She is the adaptation's best case study in expansion rather than invention: three scenes grown faithfully from one line of ink.
In the show, season by season
Season Six. In The Broken Man — Sixty-two soldiers, one unimpressed stare, zero patience.
Season Eight. In The Long Night — Killed a giant as it killed her: sixty pounds of the North's whole heart.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Fell in the Long Night (S8·E3)
The little bear of Bear Island, ten years old and fiercer than her whole hall, was seized by an undead giant in Winterfell's yard. Crushed in its fist, she drove her dragonglass into its eye with her last strength and killed it as it killed her.
Lyanna Mormont fell in the Long Night, in the battle for Winterfell against the dead — and her death is the battle's defining miniature. When an undead giant smashes through the castle's gate and begins swatting defenders aside, the smallest person in the yard charges it, screaming, dragonglass in her fist. The giant catches her, lifts her, and crushes her in its grip — and Lyanna, dying in its fist, drives her obsidian dagger into its eye and kills it as it kills her. She trades herself for a giant at ten years old, the best exchange rate any soldier achieves in the whole war; the wight that rises briefly from her afterward is put down by others, and the North's memory keeps the first version. In the books she is alive on Bear Island, her letter written, her legend not yet required.