Asumi Aida is a cheerful high school student who lives a parallel life as a player of the popular online fantasy role-playing game The World R:X, where she uses the username Sakuya. Her background is that of an ordinary teenager who was introduced to the game by her friends and classmates, Iori Ikuta and Eri Etou. In the real world, she can be somewhat shy and reserved, but while playing as Sakuya, she becomes more confident and outgoing. She is known for her kind and caring nature, consistently demonstrating a strong sense of loyalty and responsibility toward her friends. As a player, Sakuya is a skilled fighter who wields dual swords, and her avatar is modeled after a famous character from The World's history, granting her similar combat abilities.

Asumi's role in the story begins during a routine treasure-hunting adventure with her friends inside a dungeon. Their expedition inadvertently disrupts an elite guild's attempt to defeat a legendary monster known as The One Sin, causing the guild to fail and resulting in a bounty being placed on her character. While fleeing from pursuing players, she encounters a mysterious cat-like player character named Hermit. This chance meeting escalates into a much more serious situation when the group is transported to a strange area called the Digital Prison. There, Eri is player-killed and subsequently falls into a real-world coma for unknown reasons. This event transforms Asumi's simple gaming sessions into a desperate investigation. Her primary motivation shifts from casual play to solving the mystery behind her friend's coma and finding a way to wake her up. She works together with Iori, whose in-game name is Tobias, to uncover a criminal plot involving the comatose players, real-world personal data, and the motives of the elusive Hermit.

Her key relationships are central to the story. She shares a deep and enduring friendship with Eri Etou and Iori Ikuta, who are her real-life classmates and adventuring party in the game. Iori, as the shrewd and information-broker character Tobias, often takes a leading role in the investigation, while Eri, as the warrior Mary, is a dependable ally whose sudden crisis becomes the driving force of the plot. Asumi's interaction with the enigmatic Hermit is also crucial, as his search for something within the game is directly linked to the danger she and her friends encounter. Throughout the series, Asumi demonstrates development by rising to meet a serious real-world threat that manifests within the virtual world. She moves from being a casual player to a determined individual fighting to protect her friends, using her in-game skills not just for fun, but to solve a mystery with life-or-death consequences. Her notable abilities are primarily within The World R:X, where she is a capable and agile dual-blade user, skilled enough to navigate dangerous dungeons and participate in high-stakes combat against both monsters and other players. Her courage and loyalty prove to be just as important as her combat skills in overcoming the challenges she faces.