In ffx. The whole time Only Yuna could see Tidus. I think that would be pretty cool. Make it convincing that the others can see him. Till the climax where you are revealed that because Tidus was a dream. Only Yuna could see him. I think itd be cool if done properly.
do it in a way where any time you replay it. You will notice things that give away the fact he cant be seen. So they would have to be subtle. It might make no sense in combat and healing though.... so I guess maybe not. But hey its an idea I had while trying to sleep so kaboom