So, probably should have put this up sooner, but I figured there are some details about Harry's past that some students and staff might know through rumors.
1) Harry's mother: Harry's mom is rather infamous among some circles, though probably not enough for people to have heard her name unless they had some connection to the higher ups in the Ministry or the Aurors, and given virtually no one knew she had children, it'll be tricky to connect her to Harry unless he mentions her name. However, those who know her might know any number of infamous stories, which may or may not be true. She's a figure shrouded in mystery, and most of what is known is that she was an advocate of the Ministry being reformed to be more just revealing itself to the world and using its power to help solve Muggle problems, and that she had a lot of connections to a lot of shady people, especially fairies. She's also rumored to come from a very powerful magical lineage.
2) Justin DeMourne: A infamous dark wizard, while he never reached anything like Voldemort level villainy (aka threatening the world) he was still a brutal and powerful dark wizard who managed to constantly out with and outmaneuver Aurors. The most insidious aspect of Justin was his ability to blend into Muggle society, disappearing for years at a time amongst the masses, only to appear with some new violent act. Furthermore, he was not as mindlessly cruel as many of his counterparts, only engaging in acts of depravity if they somehow advanced his cause. Worst of all, DeMourne was actually a respectable Auror for many years, and no one is exactly sure when he turned to dark magic. He was thought to be dead for about a decade before his actual death in battle with Harry, and the Ministry has worked hard to brush the whole affair under the carpet. People would possibly heard of DuMourne, though he was not a household name and would be unlikely to be in any student's living memory. They might have read about him though, and might even know someone who suffered at his hands. The Ministry has suppressed both news of his recent death, and his former status as an Auror, so you're unlikely to know the former unless you're somehow connected to those who worked the case, and unlikely to know the latter unless you have government connections or were around when he went rogue.
3) The Leanansidhe: A powerful member of the Unseelie court, there are many rumors surrounding the Leanansidhe, but all of them agree on two things. The first is that she is very dangerous, and the second that she makes deals that tend to work out in her favor. No one knows of Harry's connection to her save himself and Bob.
4) Harry's trial: Harry's trial was kept hush hush, since the Ministry wanted to keep the whole affair under wraps, but was in the news. If you do some serious digging, you might find brief mention of a young arsonist who allegedly caused the deaths of two unnamed people, though there is no word on how the trial turned out, and the official records only state he was found innocent. Those with government or Auror connections are libel to know more details of the trial, namely that Harry was accused of causing the deaths of two people, Justin DeMourn and an unknown female, and was only spared from Azkaban when one Ebenezer McCoy showed up and convinced them Harry's actions were in self defense and there was no proof he killed those people.
5) Elaine Mallory: The girl who was adopted with Harry, the only information on file about her is her name and the fact that she died in the battle between Harry and Justin.
6) He Who Walks Behind: A spectral assassin, no one truly knows what it is, but all accounts agree it is bad news.