Can anyone know someone fully?

Yes, of course. But, I think this definition needs an update. In the modern day, it is very easy to meet more people in a single day than what humans were capable of in years say a couple of decades go. So, knowing someone fully, I believe, doesn’t take the literal form of a memorized snapshot of someone’s life. Instead, if you are able to almost accurately pinpoint the next course of action someone is going to take, given a situation, and do that better than the masses, you know that person fully. An important thing to notice is that you should be able to justify your choice and it should be verified by the person themself.

(PS: Thank you, Neel for asking me this question)