Kei Shindō is a student at Otobane Academy in Otowa, Japan, living as the neighbor and childhood friend of Hiro Hirono. Though one year younger, she attends the same school and routinely visits Hiro's residence each morning to wake him, ensuring he attends classes despite his late-night manga drawing. Kei excels athletically, standing out as the star player on her school's girls' basketball team even as a freshman.

Outgoing and energetic, Kei contrasts sharply with her identical twin sister, Chihiro Shindō, who is quiet and reserved. The trio was inseparable in childhood, with both sisters often competing for Hiro's attention. A pivotal event occurred when Kei was twelve; she took Hiro to the beach without telling Chihiro. Chihiro tried to follow but was struck by a car, suffering severe, lasting injuries. Consumed by guilt, Kei blamed herself and believed she became unworthy of Hiro's love, assuming he would have naturally chosen Chihiro. This guilt led her to suppress her romantic feelings.

Instead of confessing, Kei adopted a maternal role, caring for Hiro through daily routines like preparing meals. Her suppressed feelings resurfaced intensely when Hiro grew close to classmate Miyako Miyamura, whom he met during a Christmas Eve incident. Kei's jealousy manifested in actions like deleting Miyako's messages from Hiro's phone and vowing to "remove Miyako from his memory," fueling a love triangle.

Kyosuke Tsutsumi, another student, became infatuated with Kei after filming her running during Christmas. He persistently documented her daily life, and his involvement gradually helped Kei confront her unresolved emotions. In a climactic moment, Kei confessed her love to Hiro, acknowledging his choice of Miyako while finally releasing her long-held guilt over Chihiro's accident. This confession marked a turning point, allowing her emotional progress. She later developed a romantic relationship with Kyosuke.

Kei reconciled with Chihiro during a significant reunion in the narrative continuation. This interaction addressed the lingering trauma from their past and solidified their renewed sisterly bond. Outside these primary relationships, Kei is admired by fellow basketball player Mizuki Hayama, who harbors strong, sometimes fantastical admiration for her.