Kei Shidou is a supporting character who plays a crucial role in the backstory of Miki Naoki. She is a second-year student at Megurigaoka Private High School and is best described as Miki's closest friend and former classmate. Physically, Kei is depicted as a fair-skinned teenage girl with straight, shoulder-length chestnut-brown hair, which she often wears with a small section tied into a ponytail at the back. She has bright, golden-amber brown eyes and is typically seen in the standard school uniform.

Initially, Kei's personality is defined by an easy-going, cheerful, and energetic nature. According to Miki, she was always smiling and had a talent for keeping others' spirits up, often joking and acting as a mood-maker among her friends. This optimistic front was maintained even during the early stages of the zombie outbreak when she and Miki first sought refuge. However, this core personality undergoes a dramatic and tragic shift due to the immense stress of the apocalypse.

Kei's motivations and role in the story are intrinsically tied to the theme of psychological collapse under extreme duress. When the outbreak began, Kei and Miki were at a shopping mall and managed to join a group of other survivors. During this time, it is suggested that Kei developed feelings for the group's male leader, and his death in the subsequent destruction of their shelter profoundly affected her. After Kei and Miki became the sole survivors of this group, hiding alone together in a storage room, the previously cheerful Kei became deeply depressed and serious. Miki noted that she never saw her friend smile again after that day. Consumed by the hopelessness of their isolated existence and the belief that their supplies would eventually run out, Kei made the fateful decision to leave the safety of their hiding spot to seek help, even if it cost her her life. She famously questioned Miki, "Is it really enough just to be alive?" before departing and giving Miki her portable CD player as a parting gift.

The central and most defining relationship in Kei's life is with Miki Naoki. They share a deep bond of friendship and mutual care, having survived the initial chaos together. Kei’s departure is a traumatic event for Miki, who is left behind and later rescued by the main group. This separation drives much of Miki's emotional journey and guilt throughout the series. Kei's feelings for the unnamed leader of their first survivor group also serve as a key catalyst for her mental deterioration, as his death removed another source of hope and stability.

Kei's development is a tragic arc from light to despair. Her story concludes in different ways depending on the adaptation. In the manga, after leaving the mall, she managed to reach Megurigaoka Station. From there, she broadcast a desperate radio message under the name "K," reporting that she had injured her leg and was asking for help for herself and for "the girl on the top floor of the shopping mall," a clear reference to Miki. This was her final act of selflessness. Her transmissions ceased after that day. Much later, Miki encounters a zombie with a similar hairstyle to Kei's, strongly implying she was infected. In the sequel manga, it is confirmed that soldiers dispatched to the area shot and killed the infected Kei while Miki narrowly survived. In the anime adaptation, Kei’s role is slightly altered; she storms out of their hideout out of sheer frustration rather than trauma from a larger group's death, but her ultimate fate of becoming a zombie remains. While Kei possesses no supernatural or combat abilities, she demonstrates notable resourcefulness and determination, using her knowledge of electronics to set up a CD player as a distraction and operating a radio to send a final, critical distress signal.