Old man had his hand cut off by the vampires, but when he came back he seemed to remember everything. Kei's face was smashed during the Tae mission, but he also remembered everything.
We don't know anything about Izumi's first trip, but as i remember he wasn't surprised at all when he saw the 100 points menu, sooooo...

Do you think all that stuff is important? Does it matter somehow? Just give it up.

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It's self-defence, there's no problem with that!
Katsuya Suou, Persona 2 Eternal Punishment