Asuka, the older sister who lives near Shota's house. Although he never paid much attention to her in their childhood, one day Shota learns about sex from a friend and becomes increasingly curious about Asuka's big breasts. His pranks escalate—from spying on her changing clothes to sneaking peeks while she bathes—growing bolder each time, leaving Asuka increasingly flustered. What begins as innocent curiosity gradually evolves into a deeper, more intimate connection between them.