Not sure Kouichi is the
catalyst. Maybe more like a
nexus around which events are pinned? But clearly, for other people to see her, they seem to realize their time is at an end, or some similar disaster.
Dislike that Kouichi is so slow ... Mei says she's a close cousin to Misaki, used to play with and all, and he doesn't act shocked about the time discrepancy required for that to be a fact. But, then, all like "Me so shock!" when she poses "What if only you can see me?" Retard.

Also interesting that Mei's first-hand experience doesn't include knowledge of the exact way in which Misaki died. She was there, she would know. Gonna be unimpressed if these are the type loose ends the plot will ignore.
With (we assume) the death of Sakuragi (as opposed to "just" an injury), I was wondering during the end credits if
anyone other than, maybe, Kouichi will survive all the way through ... and, what 'emergency' was Sakuragi running toward ..? Damn, but that was a grim way to go ...