Youichi Yotsuba is a Japanese man with black hair and blue eyes, married to Kanami Yotsuba. His marriage faces strain due to his attraction to women with large busts and recurring infidelity. His neighborhood and social circle contain many attractive women receptive to affairs, creating persistent temptations.

He occupies the center of a complex family dynamic involving multiple women: his wife Kanami, her sisters Mia Hiragii and Kisae Yotsuba, his sister-in-law Chizuru Yotsuba, and his stepmother, the widow Kaho Akamatsu. All become pregnant simultaneously, with Kaho Akamatsu's pregnancy raising questions. This situation highlights the intricate relationships surrounding him.

His actions directly fuel the story's central conflicts and themes, particularly the shared desire among these women for motherhood, stemming from loneliness or a nurturing instinct. His interactions with them drive interconnected vignettes fulfilling their maternal aspirations. His lack of restraint and repeated infidelities generate continuous tension within his marriage and wider social circle.