my favorite headcanon for harry potter is that harry and mattheo are besties. like they're actually really tight.
it probably happened when they were forced to work on a potions project together in like 4th year and they kinda both just fucked around and ended up being chill with each other, both walking away being like "hmm...he's not so bad"
then in history of magic or smth in like 5th year they were given assigned seats next to each other in the very back and that's really when they became friends because they would both really not pay attention and just fool around the whole time.
it shocked everyone ofc bc mattheo's dad literally killed harry's parents and is actively seeking to kill harry but as the boys' friendship bloomed they actually did have a real heart-to-heart where mattheo came clean about hating his father and how he's so so horrible and harry felt so bad because he couldn't imagine having voldy as your father (and he knows what sirius's childhood was like growing up)
and they even become such good friends that when christmas break rolls around, harry invites mattheo to spend the holiday at 12 grimmuald place with harry, hermione, sirius, and the weasley's.
and harry potter, this stupid mf, doesn't tell anyone he invited mattheo so he just walks through the door after everyone is already there, laughing with mattheo by his side. immediately wands are out ofc but harry's like "no no guys he's cool. matt's cool"
and the adults (bc the kids are now used to seeing matthew and harry fooling around in class or playfully bantering on the quidditch pitch) are like "matt?????? tf you mean matt?"
(sidenote: mattheo HATES the nickname matt, but he lets harry use it)
anyways it ends up being lupin who convinces everyone to let mattheo stay, arguing that his other option of going home to voldy is far worse.
and then as the break goes on all the adults see how close matthew and harry are, and even how matthew has become friends with hermione and the weasley's that they all grow a soft spot for him.
ESPECIALLY sirius for obviously reasons bc they are so so so much alike it's uncanny. and sirius even tells mattheo that after hogwarts, he can totally movie in with sirius and harry.
basically general consensus is that matt and harry are like pure brothers. like harry loves ron and ron will always be his bestie, but matt is his brother. (sirius-james type relationship)
and it's reciprocated too, matt fully believes harry is his brother. one he def prefers over tom jr.
and even james and lily from heaven are like "aww look at our boys"
and yeah that's it they're literally so cute love the bromance