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

Then: Kunugi Kou waged bloody warfare as a hero in another world. Now: He's a perfectly average high—schooler with a perfectly exceptional best friend: Ayase Kaito. 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, Ayase Hikari, 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)