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

Then: Kou Kunugi waged bloody warfare as a hero in another world. Now: He's a perfectly average high-schooler with a perfectly exceptional best friend: Kaito Ayase. The living incarnation of every cheesy romcom protagonist ever written, Kaito's charming personality and looks that could kill have earned himself the harem of quirky beauties that every protagonist deserves—and Kou is dead-set on supporting him from the shadows as the ultimate wingman extra! And it's all going great—that is, until a wrench is thrown into the works in the form of another romcom staple: the protagonist's little sister, Hikari Ayase, who meets Kou under the most outlandish of circumstances possible. Even worse, it kinda sorta just maybe seems like she might have a thing for him? Now it's up to the world's most "extra" extra to crawl his way back into the obscurity he longs for! (Source: J-Novel Club, edited)