Here I am, writing this long series on character creation, and I haven't even attempted to define what a character is. That not necessarily bad, but it is one of those questions worth wrestling with. I'll take a stab at this, then you can laugh at me.
A character is a seeming person with different perceptions, knowledge, and decision chains that your own. Characters are infinitely repeatable.
I say that a character is a "seeming person" because a character isn't a person. Your mother is not a character, she's a real person. Sometimes a person gets called a "real character", but that's not the same thing as being a character.
Well, what about characters modeled after real persons? You've already hit the nail on the head there. Something modeled on a person is a different thing than the real person, just as the model of an airplane is not the airplane that it models. Mark Twain, the character, has had many amazing adventures that Mark Twain, the author, never did.
I also call a character a "seeming" person because a character has the sufficient traits of a real person to be mistaken for a real person. (Don't ask me what that threshold is.) I also say "seeming" because a cat or a lion can be a character and they aren't people at all, but the audience begins thinking of them as persons even if they are not, for they have enough traits that we associate with people to call them persons.
The second major part of a character is that a character isn't you. Even if you create a character based on you, that character will have an experience that you will never have.
The final bit took me a while to realize, but it's one of the critical differences between a character and a person. A character is infinitely repeatable. Each time that Hamlet considers "to be or not to be", it's the first time. Every performance of every play, he asks that question anew with no better idea of the answer than last time. So even if you create a character entirely based on yourself and your life, you can't go back an relieve that day over and over, but that's exactly what characters do. Each of their days happens anew all the time. There is no irony in the repetition.
Not only that, you can set all their adventures on the same day at the same time and there is not problem. There can be a million Mondays down by the corner store, all happening at the same time. Try that in real life.
I'm sure that they are holes in this argument. How do you treat video of yourself where you keep doing the same stupid thing over and over? I'll leave those fine points up to philosophers far better thinkers than me.