Karsi is one of the wildling leaders who retreated to Hardhome after the wildlings' defeat at the Battle of Castle Black. She lost her father, her uncle, and two brothers fighting the Night's Watch.
When Lord Commander Jon Snow and Tormund arrive at Hardhome to offer the Free Folk to retreat to Castle Black and settle south of the Wall, a meeting of the clan chieftains is held, where Karsi represents her clan. The Lord Commander's appeals to their shared purpose against the White Walkers and offer them a cache of dragonglass as a weapon against the White Walkers, which Magnar Loboda dismisses as just as unlikely as the mythical "ice spiders as big as hounds". Karsi replies that with everything they have seen none of this is unbelievable, and she decides to side with the Lord Commander's plan as long as Tormund vouches for him, but Loboda remains stubborn, claiming that all the Free Folk who get on the Crows' boats will be executed, as they have always been enemies and his ancestors would spit on him if he broke bread with a crow —though Karsi retorts that so would her ancestors, but it doesn't matter as they are dead. Nevertheless, Loboda abandons the hall and he is followed by many other chieftains, to the dismay of Jon, Tormund, and Karsi, who exclaims "I fucking hate Thenns."
When Lord Commander Jon Snow and Tormund arrive to retrieve the wildlings to escape the imminent invasion of the White Walkers, Karsi initially is hostile towards Jon, but after listening to him and Tormund, she agrees to their plan and begins boarding her people into longboats.
They begin boarding the wildlings into the longboats, and Karsi puts her daughters in one of them, telling her youngest that her older sister Johnna is in charge until she comes back. The girl protests, but Karsi says she needs to be there to get the old people on the boats, and promises to be just behind them. Then, the White Walkers attack with their Army of the Dead, and Karsi joins the defense of Hardhome while buying time for the rest of the Free Folk to board the boats bound for Castle Black. During the Massacre at Hardhome, Karsi manages to kill several wights while defending the remnants of the Free Folk. Jon Snow tries to convince her to board the boats, but Karsi argues he should go instead, as she doesn't believe the Night's Watch men at Castle Black will let her daughters and the rest of the wildlings through the Wall if Jon Snow is not there. During the fighting, Karsi is confronted by child wights and freezes in shock. Unable to bring herself to kill children, she is overwhelmed. A moment later, she becomes part of their army raised by the Night's King.
While Bran Stark is on the run from the undead army of the Night King, Karsi is briefly shown as a wight in one of his visions.
Karsi was an Ice River elder living with her two children at Hardhome. She was killed by wights during the wildling massacre and is now a soldier in the army of dead.