Winter was Leia Organa’s adopted sister in several Legends books and comics. She was the daughter of Bail Organa’s aide Sheltay Retrac. After Retrac’s death Bail and Breha Organa adopted Winter and raised her with Leia. Winter fought with the Rebel Alliance for years and was an operative for Alliance Intelligence.
Author Timothy Thrawn created the character Winter in his Thrawn book trilogy that came out in the 1990’s. She went on to appear in many books and comics (see a full list on her Wookieepedia page).
Check out the entire list of women in 365 Days of Star Wars Women in the Women in Star Wars Index. It includes highlights from each post plus notes which posts include new 365 interviews with actresses, writers, artists, and more.