Sanbante Heroine no Sonogo

Synopsis

"I lost. I can't be his girlfriend (heroine)." Natsuki Riara, a bold, sexy, and curvaceous third-year high school girl, is widely acknowledged as "the third-best girl at school," but she is rejected by Kazuki Shikimura, the boy she has a crush on. When Kazuki's younger brother, Senri Shikimura, tries to comforts her, Riara jokingly says, "Maybe I should just go out with you instead." She is surprised when Senri agrees and confesses to her, "I've always loved you, Natsuki-san..."? (Source: Tonari no Young Jump, translated)