Harry was on his way to his first year at Hogwarts.
Seated with him, in his compartment, was a red-head named Ron Weasley and an oddly knowledgeable young man who introduced himself as Roman Ravenclaw (a name that seemed to impress Ron even more than Harry's own).
Roman was weird; he didn't act like a child at all. When Ron had excitedly asked to see Harry's scar, Roman had dressed him down saying that Harry wouldn't want to be reminded of the night his parents died. This, Harry thought, wasn't fair to Ron because Harry really had no problem showing his scar to Ron and he hadn't thought of his parents' murder until Roman brought it up.
Talking to the two boys was made even more uncomfortable when Roman started taking Ron to task because his family doesn't talk to an accountant uncle he has. Harry wasn't sure that the Weasleys were being discriminatory or if there was more at play in the scenario, but either way, it wasn't Ron's fault.
Harry also didn't like the way Roman kept looking at him whenever his muggle upbringing came up. He made it clear he didn't think there was anything wrong with most muggles, but Harry still got the impression that Roman thought very little of the Dursleys (which, fair enough, they are awful, but it wasn't like Roman could know that).
When a bossy girl showed up looking for a toad, Roman lectured her on trying to make better first impressions (as though he had any room to talk) and suggested she ask a prefect to summon the amphibian.
When Harry bought one of every sweet from the trolley, Roman looked disapproving, but didn't say anything, perhaps becoming unaware that Harry didn't have anything else to eat.
Then Draco Malfoy showed up.
Malfoy started by barging in without knocking. Then he immediately started insulting Ron while being oddly respectful of Roman (at least once he learned Roman's last name). Then he stuck out his hand like he expected Harry to shake it.
"I think I can figure out who the wrong sort are for myself, thank."
Draco's face pinked and went to pull his hand back, but Roman spoke up.
"Excuse him, Heir Malfoy; he has been brought up in the muggle world and doesn't yet understand the social implications of his actions. Harry, deliberately snubbing the hand shake of a peer is a grave insult in pure-blood society. Even if you find his politics distasteful, you still shake his hand. Feuds have been started over less." As Roman said this, Harry decided he couldn't stay quiet any longer.
"Okay, first of all, I could not care less about the social niceties of pure-blood society. In case you weren't aware, I'm not a pure-blood. I don't care if some inbred little racist gets butt-hurt that I refuse to touch him. Second, you have got to be one of the most obnoxious people I have ever met. You have spent this entire train ride lecturing everyone you have come across for little things, but when the outright bigot comes in insulting my friend, you act like I'm the one out of line for not literally shaking his hand. Let me make this clear: we are all eleven. I am eleven, Ron is eleven, Hermione was eleven, Malfoy over there is eleven, and you are eleven. Stop trying so damn hard to pretend that you are an adult and that it matters so much if we are accidentally a little rude to each other."
A.N. So this is in response to every story about Harry being taught proper noble etiquette for pure blood society and also in response to stories with self-insert characters trying to fix everything right away.
