Katelyn is the youngest child of the Estermonts, born after a failed pregnancy which made Jeyne particularly attached to her. She is described as yellow haired like her mother, with her green eyes to match.
Katelyn accompanied her mother to King's Landing after the death of Loren Lannister, who had been her uncle.
Katelyn accompanied her mother to Casterly Rock after Jeyne was named acting Wardeness of the West by King Damon Lannister.
Katelyn attended the Tournament of Three Ships in the Westerlands without her mother's consent, sneaking to Fair Isle with two lowborn friends whose parents were in the employ of the Rock. After they were caught, Lady Jeyne accused the oldest of her companions, a boy by the name of Jate, of abducting Katelyn based on the testimony of a knight by the name of Ser Victor Estren. In the subsequent trial by combat, Ser Benfred Tanner (as the champion of Jate) slew Ser Gunthor Lannister.
"Katelyn was her youngest, and after the loss of the son who would have come before her, Jeyne kept the girl close, closer than Aemon ever approved of." - Blood and Whispers
"She will never grow to be independent, if you continue to coddle her like that. You do her no favors with your doting." - Aemon Estermont
“I’ve spent enough time with Lannisters to know one when I see one. Maiden of sixteen, hair of gold, eyes of green, saying something about Greenstone being fairer than Fair Isle itself - brags like the lot of you, I’ll tell you that.” - Benfred Tanner
Aemon Estermont, father
Jeyne Estermont, mother
Martin Estermont, brother (deceased)
Willas Estermont, brother
Elena Estermont, sister
Bennet Estermont, brother
Eldon Estermont, brother