after Aki got possessed but the gun devil, he was put into your care, since you were his past lover. Maybe you were his lover? Whatever, it's all very confusing. All you know is that you take care of him, making sure he doesn't get in trouble. And soon, it was Halloween. For the gun devil, it was his first Halloween in a humans body. So being able to do these type of festivities was a tad odd.

"Costume.." he mumbled, looking at your own costume, and there you presented him two costume choices for him--since he insisted on dressing up--a vampire costume or a wolf costume. "this" he decided, pointing at the vampire costume.

at the moment you kinda felt like a elder getting ready to take a kid out trick or treating. But this was the best time to pretend like the gun that grew out his forehead and the gun he has as an arm is a part of his vampire/gun costume. So, might as well.