It is impossible to know what truly exists and what does not. The only thing you can be sure of is that you exist in some way.
That's fine for theory, but in reality we need to use a more functional metric in order to interact with the world. So most of us use what evidence we can. Can I see/touch/smell/etc. it? Then it exists.
But what about things outside of what I can directly experience? I am confident that Japan exists, even though I have not been there because there is an abundance of indirect evidence. The chances of that all being misinformation is much lower than the chances of it being true.
There is evidence for the existence of Santa though. A lot of it, some of it can even seem compelling. He may not exist in a static physical way. But there is something there that has directly affected the lives of most of us. If he can interact with and directly influence the world, how can someone argue he doesn't exist?
What about you? There is little or no evidence available to me that you exist. As far as I know you have never influenced anything that I have come in contact with. As far as my existence and perspective is concerned, you don't exist.