Marasiah Fel was the Empress of the Fel Empire. She appeared in over a dozen Star Wars: Legacy comics (which are now all considered Legends). Before she became Empress, Fel was trained as an Imperial Knight as she had a strong connection to the Force.
You can learn more about Imperials Knights from this video from Star Wars Explained.
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.