"Professor," [Harry] started again after a moment. "The Sorting Hat told me I'd – I'd have done well in Slytherin. Everyone thought I was Slytherin's heir for a while…because I can speak Parseltongue…"
"You can speak Parseltongue, Harry," said Dumbledore calmly, "because Lord Voldemort – who is the last remaining descendant of Salazar Slytherin – can speak Parseltongue. Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure…"
"Voldemort put a bit of himself in me?" Harry said, thunderstruck. […] "It only put me in Gryffindor," said Harry in a defeated voice, "because I asked not to go in Slytherin…"
"Exactly," said Dumbledore, beaming once more. "Which makes you very different from Tom Riddle. It is our choices, Harry, that show what we truly are, far more than our abilities." (18.54-61)
In this discussion between Harry and Professor Dumbledore, Harry's
choice between Gryffindor and Slytherin seems to mean the same thing as a
choice between good and evil. Harry chooses to align himself with
Gryffindor, so he is not a Slytherin. What if you don't know what you're
choosing when you're Sorted, though? Do you think it would be possible
to be placed in the wrong Hogwarts House? What would happen if you
became a totally different person from 11 to 17 (which, after all, isn't
that unusual)? Could you outgrow your Hogwarts House?