Eichi Sakurai is a pivotal supporting character whose presence drives the central narrative of the story. He is introduced as the childhood friend and first love of the protagonist, Mitsuki Koyama. The two met while living in the same orphanage; at the time, Eichi was a quiet boy who harbored a deep resentment for the moon, as it reminded him of the night his grandfather died and witnessed his grief. However, his perspective changed entirely upon meeting the young Mitsuki. He came to compare her smile and resilient personality to the moon, describing her as someone who shines brightly while hiding her wounds, and from that point on, he grew to love the moon.

In terms of appearance, Eichi is depicted as a tall boy with golden-brown hair and brown eyes. His personality is defined by a gentle, kind, and genuinely affectionate nature. According to Mitsuki, he was the first person to ever show her kindness. He developed a strong passion for astronomy, a dream he shared with Mitsuki, who herself loved music. The two encouraged each other, with Eichi promising to become an astronomer while supporting Mitsuki’s dream of becoming a singer. His love for astronomy was directly linked to his love for Mitsuki, as the moon became a symbol of her presence.

Eichi’s role in the story is that of the objective and the emotional core of the plot. The entire narrative begins because Mitsuki believes that by becoming a famous singer, her music will reach him in America, fulfilling a promise they made to achieve their dreams before reuniting. Tragically, it is eventually discovered that Eichi passed away shortly after moving to the United States. In the anime adaptation, he died at the age of sixteen from a car accident that occurred roughly two months after his departure. Mitsuki was unaware of his death for two years, spending that time striving to fulfill her dream for him.

The key relationship in Eichi’s life is, without question, his bond with Mitsuki. Their connection evolved from a close friendship, similar to siblings, into a deeper romantic love. Over time, Eichi realized his feelings for her and confessed, though circumstances of his impending move to America complicated their farewell. He left for America believing he may have burdened her, but he never stopped loving her. Even after his death, his love remained strong. In the context of the supernatural elements of the plot, Eichi’s soul refused to pass on. He remained on Earth as a spirit, watching over Mitsuki. Unbeknownst to her for most of the story, his lingering presence is what allowed her to see the shinigami, Takuto and Meroko.

Eichi’s development is largely experienced through flashbacks and his influence on Mitsuki’s emotional journey. While he is a static character due to his death occurring before the main timeline, his role evolves from a distant memory of a living boy to a protective spirit. He struggles to let go of his attachment to Mitsuki, but as she finds happiness, new dreams, and the will to live beyond her illness, he is finally able to smile and disappear, accepting that she will be okay without him. Regarding notable abilities, Eichi is portrayed as a normal human boy without any supernatural skills. However, in his spiritual form, his presence acts as a lens that allows the protagonist to see otherwise invisible death gods. In the manga continuity, his soul also protects Mitsuki from death itself, altering her fate.