Keigo Sayama is a student in Class 2-B who perpetually wears a medical mask. This habit stems from a childhood trauma where his words unintentionally caused classmate Kobayashi to cry, leading him to associate speech with harm. The mask serves as his physical barrier against causing further pain and forming emotional connections.

Outwardly perceived as a grouchy loner actively avoiding interaction, Keigo possesses a fundamentally kind and gentle nature. He expresses care through quiet actions like helping others, though his aloof demeanor often obscures these gestures. Academically, he struggles significantly, especially with exams, creating a need for tutoring.

His life shifts when popular classmate Tsuzuru Saikawa takes an unexpected interest in him. Facing academic failure and his mother's threat to confiscate his beloved video games, Keigo reluctantly accepts Saikawa's tutoring offer. Their arrangement includes an unconventional condition: Saikawa may explore the area beneath Keigo's mask through touch, focusing on his mouth, while the mask stays on. This forced proximity sparks a complex relationship. Despite initial resistance and discomfort from early non-consensual interactions, genuine romantic feelings gradually emerge between them.

This relationship catalyzes Keigo's slow personal growth. While retaining his core introverted and analytical personality—manifested in quiet observation and logical reasoning over emotional expression—he becomes less isolated. He starts engaging more with classmates, participates in activities like the senior trip committee, and even delivers a campaign speech for student council candidate Yoshida, signaling a cautious opening up, though he remains characteristically reserved.

Beyond his mask and social struggles, Keigo enjoys typical leisure activities like video games and manga. He lives with his mother and older sister, Chie. Physically, he has auburn hair, orange eyes, and a distinguishing beauty mark near his mouth.