Aunt Liss is the mother's sister of Mayelbridwen Singer, who is one of the protagonists of The Crown of Dalemark. We don't know much about her, except that she is kind, loving, and runs a livery stable. As a result, she is able to keep her niece in food, clothing, and horses to ride, all of which are things that Maewen's mother would probably forget to provide for her, being an absent-minded artistic type.
The only other detail we learn of Aunt Liss is that when Maewen is going around the Tannoreth Palace in Kernsburgh, on her first away-from-home visit to her father, Maewen notices a painting of an historical figure known as Enblith the Fair, who seems to Maewen to be insufficiently fair, and who in fact reminds her of no one so much as her Aunt Liss.
Aunt Liss puts Maewen on the train from her home in Adenmouth to go to Kernsburgh, and when she does so, she takes special pains to ask a young man who seems to be some kind of railroad employee if he would please keep an eye on her niece during the journey. He agrees to do so. That person turns out to be Wend Orilson, who figures later as her father's assistant at the palace, and a mysterious, powerful figure, in the part of Dalemark's past that Maewen is fated to travel to.