The protagonist begins as an herbivore: passive, afraid of rejection, consuming media instead of participating in life. He desires the nikushoku (carnivorous) energy of the kuro gal —her ability to take what she wants from the world.
The manga is available in English under titles like F---ed by My Best Friend on platforms like Coolmic. Themes and Style I Became a Black Gal So I Fucked My Best Friend - IMDb Kuro Gal ni Natta kara Shinyuu to Shite Mita