Backstage is the 14th chapter of Ajin: Demi-Human, created by Gamon Sakurai.
On the day Kei escaped Tosaki group were forced to watch through the cameras. Tosaki tried asking Colonel Kouma to deploy the Anti-Ajin to stop them but was declined. Tosaki pointed out that Sato wasn't using his IBM and that this could be there chance to stop him and before Ogura could confirm Tosaki's theory he was stopped by his bodyguard.
While Tosaki wondered around he found the trail left behind by the surgeons and discovers one of Sato's guns. Now desperate he picks up the gun and uses it to kill Ogura's bodyguard and kidnaps Ogura leaving behind an unrecognisable corpse.
Tosaki later visits the hospital where his comatose fiancée is staying resolving himself to save her regardless of whoever else he sends to hell.
Kei finally washes ashore somewhere commenting on how he isn't used to helping people.