Kuso Ero Gaki Otouto Ni Ubawareta Kanojo Rj011 Top ((top))

While the protagonist tries to nurture their bond slowly, his younger brother—the titular "naughty little brother" (ero gaki)—takes a much more aggressive approach. Exploiting the protagonist's hesitation and Haruka's vulnerability, the brother begins to systematically seduce her, leading to a series of escalating encounters.