Plot Twist: In the same vein as the "Yasu is the murderer" meme, the Buff Greaser is actually a highly trained assassin who killed his accomplice, his own twin brother, at the request of his client, the Bellboy. However, the Bellboy violates their contract which leads to the Buff Greaser to successfully tricking Yumihiko into fingering the Bellboy as the killer.

After the case, the Buff Greaser then joins Yumihiko as his unofficial assistant in order to manipulate him so the Buff Greaser can cover his own tracks as an assassin.

Oh. My. Gawd. I think we're onto something, anon!