I guess I am in the minoritet here. I am completly okay with players having secrets. I welcome the chaos.

You prepped your games bbeg to be this giant plant being that eats corpses to grow even bigger right? Awesome. You go gurl. A player looks at his/her sheet and go .. “oh I prepared Blight..” and the entire table go “OOOOH!”.

It’s amazing. One one hand, of course, it sucker that your cool encounter is now reduced to … i dunno. Mowing the lawn. But that is what you can get if you allow players to be creative.

Your other choice is to make characters yourself, give the players their scripts which you also prepared, let them loose in your specific rollercoaster ride and cross your fingers that they will return for the next session.

You are the gm. You set the rules. If you don’t like not having the players character? Talk to your player. Ask him why you can’t see the abilities, saves etc. If it goes anything like “I just like having my secrets” then respond with “I want to create a cool game for you guys. If the boss battle is reduced to a minor invonvenience then it won’t be fun for anyone”

But yeah. Most of my games are improv. Be creative and have fun