Ayumi Sakaguchi catches Kiyoi Mizushima's eye during a classroom glance. Possessing a cheerful and bright personality that makes her popular, Ayumi admits to being pushy and stubborn. She repeatedly tries to befriend Kiyoi based on their mutual interest in the WIXOSS Novel, offering to lend Kiyoi the second book and challenging her to WIXOSS battles to build their friendship.

Her persistence finally wears Kiyoi down to a single game. However, after Kiyoi builds a deck and meets Ayumi's friends, distrust sets in. As Kiyoi tries to leave, Ayumi pushes her out of the path of an oncoming truck. This heroic shove lands Ayumi in a coma. While hospitalized, the entity Remember, inhabiting Kiyoi's body during Selector battles, disconnects Ayumi's life support, causing her death. Remember frames this act as "saving" Ayumi from suffering, leaving Kiyoi devastated.

Ayumi has long hair typically worn in a left-side ponytail. Her surname means "slope, hill" and "mouth, entrance," while her given name means "walk" and "beautiful." She claims the title of strongest WIXOSS player in her class and wields a Red/Blue deck featuring Weapon and Aquatic Beast SIGNI.

Ayumi's legacy deeply shapes Kiyoi's journey. Ayumi once remarked Kiyoi would look cuter with short hair, a comment that later motivates Kiyoi to cut her hair. Kiyoi also visits Ayumi's grave with Amika Hashimoto, a girl bearing a resemblance to Ayumi, leaving a flipped LRIG card as a tribute. Ayumi's death fuels Kiyoi's motivations in ensuing conflicts, becoming a symbol of lost innocence and the harsh consequences of the Selector battles.