Shinyuu Mob no Ore ni Shujinkou no Imouto ga Horeru Wake ga Nai

Kunugi Kou was once a warrior in another world but is now a normal high school student with his best friend, Ayase Kaito. Kaito is popular, good-looking, and has a group of admirers. Kou is content being his supportive friend in the background. However, this changes when Kaito's younger sister, Ayase Hikari, appears to have feelings for Kou. Now, Kou must deal with this unexpected development and try to return to his quiet, unnoticed role.