It seems quite a few people are misinterpreting the ending, so here I am with a little explanation.

Henry did not get his memory back. Remember when Zhou Mi said that he had good days and bad days? He was just having a good day and remembered Zhou Mi and everyone else at the ward because of that. But just for that particular day, which is why Zhou Mi said at the end that he knew Henry wouldn't know who he was in the morning, because he probably wouldn't remember two days in a row. That is also why Henry didn't apologize to treating Zhou Mi badly the night before, because he didn't remember it.

Also, all the patients did not get better. Because Henry remembered them for that one day, they all tried to return to how they used to be in order to savour having him around again and to not worry him, because he still had no idea about his disease even though he remembered who they were. So the next day when they wake up and find Henry doesn't know them again, it will all start over and they will all withdraw back into their depression.

And lastly, he is whoever you want him to be. He could be just a normal average guy who fell in love with a stranger, or he can be your favorite kpop idol from any group you want. Or he could even be you yourselves. I wanted to leave it open so that he wasn't a difinitive person and so that you could imagine it yourselves.

Thanks for reading, I hope this clears up some of the confusion! If anyone has any more questions feel free to ask me!