Eurogamer is reporting a terrible glitch in Nioh. Some gamers are reporting their old characters are deleted when they start a new one, and it is not always immediate. You can check out the threads on Neogaf and Reddit to see other players who have already been hit with this unfortunate problem. One player had a level 98 character vanish when he started a new one. Ouch.
With the difficulty in Nioh, no one wants their hours of effort to disintegrate. The game allows for multiple characters, so, unless the developers thought their game was so difficult players would only subject themselves to it once, it makes sense that this would have been tested. If nothing else, hopefully, they can put out a patch to fix the issue.
Until then, keep your family, significant others, and everyone else away from your PS4 until you can backup your save.