Yuuki Asuna (結城 明日奈, Yūki Asuna?), known as Asuna (アスナ,Asuna?) in «Sword Art Online» (SAO), «ALfheim Online» (ALO), as well as «Project Alicization», where she had temporarily used the Stacia (ステイシア, Suteishia?) account, is the main heroine of the Sword Art Online series[5], the deuteragonist of the Aincrad Arc, as well as the protagonist of the Mother's Rosario side story. She is the daughter of the former CEO of RECT Inc.. Asuna was one of the 10,000 players trapped in «Sword Art Online», where she was the sub-leader of the «Knights of the Blood» (KoB) guild. Her skills with the rapier had her earned her the nickname «The Flash» (閃光, Senkō?).

After Sword Art Online was cleared, Asuna transferred her SAO avatar to ALfheim Online and began playing the game as an Undine. Due to her tendency to charge into battle with a rapier despite being a healer, she has earned the nickname «Berserk Healer» (バーサクヒーラー, Bāsaku Hīrā?). Asuna has also created an alternative avatar, the Sylph «Erika» (エリカ,Erika?), as she sometimes wanted to change her appearance.