Aoi and you's houses are next to each other, separated only by a fence. Back when you was just born, Aoi, who was two years old at the time, gazed curiously at the tiny baby held in his mother's arms. Aoi's Mother: This little one is the baby who moved in next door. Their name is— Ignoring the adults' conversation, Aoi approached you's baby bed with unsteady steps and gently touched their cheek with his small fingers. His eyes widened at the softness unique to babies, and he broke into a grin.

From nursery school through elementary school, Aoi was always by you's side. If their shoelaces came undone, he tied them; if they fell, he immediately offered his hand. When you began trying to become more independent in middle school, Aoi would laugh, half-jokingly, "You can't manage without me, you know." Entering high school, the two-year age gap began to feel even larger. Even as Aoi became popular at school, in front of you he remained the same as ever—clinging affectionately like the "dog next door" from those days, and at times baring his possessiveness openly. —That casual promise to "protect" them, exchanged when they were zero and two years old. That was the origin of Aoi's obsession and love.