Koito Nagase is a character central to the plot of Wonder Egg Priority. She was a transfer student who became the first and only close friend of the protagonist, Ai Ohto. Koito has brown hair that reaches her chest and wears glasses. She is portrayed as highly calm and mature, unfazed by bullying and consistently able to find good aspects in negative situations. She is kind and gentle, particularly toward the people she cares for.

Koito's background is marked by significant internal struggle. While externally composed, she harbored a deep fear of being unwanted or unloved, driving a core desire for genuine connection. The reasons behind her suicide, which involves jumping from the roof of the school, are a central mystery of the narrative. The story strongly suggests that relentless bullying from classmates and a complicated relationship with a teacher named Mr. Sawaki were contributing factors.

Her role in the story is that of the emotional linchpin. Her death is the inciting incident that leads Ai into the surreal Wonder Egg dream world in an attempt to revive her. Koito's memory guides Ai's actions, and the mission to save her is the primary motivation for the protagonist. Koito's trauma heavily influences the dream world, which takes the shape of the school where she died.

The most significant relationship in her life is with Ai Ohto. Koito sought out the isolated Ai and formed a deep bond with her, becoming a source of light in Ai's life. The story heavily implies that Koito's relationship with Mr. Sawaki was a source of deep pain and betrayal, which formed a crucial part of the secret she carried.

Koito's character development is primarily explored through flashbacks and evolving revelations about her final days, rather than through present-day action. She does not fight or possess combat abilities; instead, her narrative power comes from her status as the girl Ai is trying to save. Her story highlights the series themes of suicide, the complexity of trauma, and the nature of friendship and sacrifice.