Can you ever really know somebody?

Since we're always looking at people through our own clouded vision and our own interpretation and perceptions, do we ever really see who someone is? Can we truly see who we, ourselves, are? Or, is the way we see things really the truth? Maybe the way we see things is really how it is. Maybe there isn't a deeper level of understanding or something hidden under the surface that we can't see. How do we ever really know what the truth is and who someone is...including ourselves? One person sees him/herself as an extrovert. A friend sees that person as an introvert. Who's right? Does the person's view of him/herself trump what the friend sees? Or does the friend's view overrule the person's view? Or are they both right somehow?

Now, you may be thinking that there isn't really a point in thinking about things like this. But, this applies to so many things in life:
When you're trying to change something about yourself, it's helpful to know at what point you're starting;
When you're trying to determine why someone has done something (e.g. acted a way that seems "out of character");
When you're trying to predict what someone will do or how they will react to something;
When making promises, you base your promises off of what you think you will do sometime in the future. What you think you will do is based off of past actions as well as how you think you will react (or WOULD LIKE TO react);

Anyway, I'm wasting time.