Harry Potter, abused, afraid and alone ends up wondering around King's Cross Station, looking for Platform 9 3/4. Hagrid, no matter how nice the gentle giant is, forgot to tell him how to get on the Hogwarts Express, but over-hearing a rather large family of red-heads talk about the Platform and Muggles, he follows them. His extreme shyness and distrust and blatant fear of most adults has him steer clear from asking for assistance.
After getting on the train, his world is thrown through the proverbial ringer, but he gets assistance from the comfort of a kind and gentle girl who is also starting at Hogwarts. Now let's start on a more truthful journey of how Harry Potter would've acted.
(Slytherin Harry Potter) There are many moments in our lives that add up to define who we become, and some things that we simply are with or without any choice in the matter. Getting to Platform 9 3/4 by himself and finding a toad roaming the train, one small moment among thousands, really should not matter all that much in the grand scheme of things... Except it does.
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Or, Harry never meets Ron on the the train to Hogwarts and goes to Slytherin instead. This changes everything that it can.