Eri Shinkai is a wealthy individual from an affluent family who uses her considerable resources to pursue a personal passion for mysteries and adventures. As class president, she establishes her school's Treasure Hunting Club. Her deep fascination with enigmas leads her to recruit Aika Sumeragi, specifically to leverage Aika's Class C salvager license for an investigation into their classmate, Karen Minamino. This inquiry focuses on a coded message hidden within a tattoo on Karen's body.

She possesses long brown hair, which she typically wears in twin tails. Her usual outfit is a purple top, a yellow neckerchief, and a light purple miniskirt. She displays a pronounced confidence in her physical appearance and shows no embarrassment in situations involving nudity, a trait that often contrasts with the reactions of her peers.

Her personality is curious and can be occasionally selfish. She takes pleasure in narrating her adventures aloud and frequently engages in heated arguments with Aika Sumeragi, though this contentious relationship gradually matures into a firm friendship.

Years later, she enrolls as a university student and reunites with Aika Sumeragi. She becomes involved in an investigation at a private Catholic school where students are vanishing under mysterious circumstances, during which she provides essential equipment and support. She also engages in financial activities, including stock trades connected to her family's company.

Her background grants her access to significant resources, including a private cruiser that is operated by a crew, one of whom is a captain skilled in operating a sniper rifle.