The culprit is Masashi Watabiki. He actually had a twin brother, which accounts for how he managed to commit his murders all the while having a perfectly valid alibi. He reveals he didn't want to kill Atsuhiro Kunitomo, the latter died of a heart stroke in front of him due to Watabiki being too late to give Kunitomo his medicine. Kunitomo had suddenly felt bad since he thought Watabiki's twin was a ghost. After he died, the Watabiki twin hid behind the room's door, and as everyone rushed in to rescue Kunitomo, he melted into the group, while the other twin stayed behind and left.
