Disclaimer: Last time I checked, I wasn't Nagaru Tanigawa, so I own nothing.
"You're not the real one."
The not-real Kyon on top of him just smiled wickedly. Staring into Koizumi's eyes, he just said:
"I'm the closest you'll ever get to having him"
Rage almost burned him up. Rage that came from denial, from neglect, from every single piece of what had been his secret hope, now crushed because of God knew what speaking through Kyon's mouth, the one which had been in his merely seconds ago.
He restrained himself. He had to. He couldn't afford losing grasp of reality. Not here, not now, with the authentic Kyon being just the next door. Some ghost that had came from nowhere wouldn't make him risk everything.
Therefore, he just stared back and said, coldly:
"Out of my way."
One shove, and the not-Kyon was off him and on the floor, looking bewildered and angry.
Without paying him attention, Koizumi rushed to the door, but faltered as he was about to open it. It was overwhelming, all the thoughts of regret.
Just once... He won't know about, he doesn't have to... You could get to know... He's right, this is probably your only chance...
Shaking his head and without looking back, he grabbed the knob, opened and shut the door behind him, maybe harder than it was necessary. He blinked, surprised as to find everyone standing by their respective doors, looking as shaken as he was surely looking right now.
Ah, so he wouldn't be the only one regretting things. That would be enough to help him get back his smiling façade without no one noticing, which did almost at once.
No one would ever know the details of that night, nor the little hope that he still had.