Lyanna Mormont

Lyanna Mormont is a recurring character in the sixth season, who was previously mentioned in Season 5. She is the 10-year-old Lady of Bear Island making her head of House Mormont of Bear Island. She is the niece of Lord Commander Jeor Mormont of the Night's Watch and cousin of Ser Jorah Mormont.

Biography
She is the daughter of Maege Mormont. She was named after Lyanna Stark, younger sister of Lord Eddard Stark, who died years before she was born. After her mother's older brother Jeor joined the Night's Watch and later his only son Jorah fled to Essos to avoid a death sentence by Lord Eddard Stark upon being caught selling poachers into slavery to fund his wife's expensive tastes, Maege Mormont became Lady of Bear Island.

Season 5
Stannis Baratheon has come to Castle Black and send ravens to all the noble houses of the North with messages asking them to join his cause. Lyanna Mormont sends back a rebuff that she and Bear Island only bend the knee to House Stark. An upset Stannis shows Lyanna Mormont's message to Jon Snow which causes Jon to briefly smile and point out that, much like the Free Folk, the Northmen are loyal to their own.

In the books
In the A Song of Ice and Fire novels, Lyanna Mormont is the youngest of Maege's five daughters, and the only one to remain at Bear Island during the War of the Five Kings. She is ten years old at the time Stannis arrives in the North. Just as in Season 5 of the TV series, Lyanna has not physically appeared so far in the novels, and is only mentioned during the scene in which Stannis shows Jon Snow the letter that Lyanna sent him. Lyanna's mother Maege appeared as a non-speaking cameo in Season 1 of the TV series.

In the novels, Stannis is similarly unaware who Lyanna Mormont is when he receives her written rebuff, and asks Jon Snow to identify her. As in the TV version, Jon says to Stannis that she is the Lady of Bear Island - though inwardly Jon thinks to himself that she actually isn't, because she has older sisters, and wonders why she would be identified as such in the letter.

In the novels, Maege is still alive, and was not present for the Red Wedding because Robb sent her on ahead to the Crannogmen of House Reed in the Neck, to prepare them for a push to retake Moat Cailin from the ironborn. Her whereabouts after the Red Wedding are unknown. Maege's eldest daughter Dacey was a member of Robb's personal bodyguard, however, and she was killed at the Red Wedding. The exact situation in the TV series is unclear, but this doesn't necessarily mean that Maege died off-screen in the TV version: simply that Lyanna is the acting Lady of Bear Island because she was physically left in charge there, just as Bran Stark was the acting "Lord of Winterfell" during Season 2 due to Robb's absence.