Well, it is not physically possible to fully understand someone, as you can never know every memory and influence on someone. People are just too complex to fully understand them. I did mean something more along the lines of your description of not-wearing-masks.
The only reasons I can think of for fully knowing someone, is to appreciate them and to communicate with them better. Those two factors don't necessarily require you to know everything about someone. A generous amount will do.
Maybe it's better to remove the past from the equation. The full picture I seek in people is the unrestrained existence of a person. The way they act (or more literally, how they don't act but are) around me, without masks. So let me rephrase: I think I only truly know someone when they are comfortable enough to show their unmasked self, which I am familiar with. Just like you and your SO. I am grateful to have people around me who fit that criteria. Circles of friends where I can just be my true self. I feel like I have the full picture of those people, as I understand how they think because I've had plenty of conversations with their unmasked self.
_refugee_, isn't that truly knowing someone? Pursuing perfectly knowing someone is futile, after all.