I'm writing a Marauder Era fic in which a Slytherin and Remus Lupin, not too quickly, but fairly easily become friends. What I'm not positive about is the significance of the difference in house. I remember reading that the the severity of house rivalries always fluctuated over the years but I don't even know where to begin looking for that information.
Considering the hostilities between Snape and the Marauders, and the fact that the Deatheaters were organized and/or became active almost immediately upon the Marauders leaving Hogwarts, I am not clear on what part parentage would play in chosing one's friends while at school those years.
I've continued writing as though at that time, families such as the Malfoys and Blacks were considered extremists and that it wasn't so unheard of for students in different houses -even a Gryffindor and Slytherin to be good friends.
Anyway, hoping for some imput and different perspectives.
Another thing I've taken into account is that my original character is female. And with the exception of Bellatrix who I'm sure gladly enlisted if not took a large part in the organization of the Deatheaters, I don't know that many pureblooded daughters are given the choice to join. I may be wrong about that, too. I'm just back from a very long fanfic break :b
I think that you would need a really plausible reason for Lupin and Snape becoming friends because Lupin is friends with Snape's worst enemy. The houses probably wouldn't have as much affect on their friendship but I think that Snape's thinking Purebloods are supreme might.
Most pureblood families did support Voldemort but they didn't go as far as becoming Death Eaters. I would think however that some parents might be proud that their children had the guts to serve Voldemort. This is what had happened with Sirius' brother. Other families might be protective of their kids in which case I could definately see the child becoming a Death Eater against their parents wishes.
Hope that helped.
Snape isn't actually the Slytherin I was referring to. My OC befriends Remus. A Regulus sort of girl, in that where her family stands @muggles and the interbreeding of half-bloods/muggle-borns/pureblooded wizards, she isn't even sure why it all matters... but it will all eventually boil down to a question of loyalty.
Snape Hates Me
I don't think having the cross-house friendship would be significant on it's own. Lily after all was best friends with Snape.
This is Remus and I don't think James and Sirius would like Remus being friends with a Slytherin. Most of the school would think it was a bit odd, and shrug it off. Remus may run into a bit of trouble with his friends.
The fact that it is a girl may make it easier for James and Sirius to accept. Maybe they could think Remus was just having a fling with her and they don't think they have an intellectual bond. They could go along with it for a bit, before they realize they actually friends. Then they may try and bust up the friendship or something.
The girl of course would be pressured by her friends "You could do so much better." "Why are you hanging out with that Gryffindor slime?" I'm not sure if they are just friends or you are involving them romantically, but it works the same.
Of course if this isn't a main component of your story, your answer is yes, the friendship is plausible.
If it is the major component, you could add in all the factors I've mentioned above.
Hope that helped
Of course, when you say "house rivalry," what you really mean is "rivalry between Slytherin and the other houses." Because in the series, the rivalry between the other three houses remains mostly friendly, and we see plenty of instances of Gryffindors, Hufflepuffs, and Ravenclaws being friends, dating and marrying each other, etc.
It's always Slytherin house that's the issue. (A big problem I had with DH was the way that Rowling never resolved that.)
Presumably, Slytherin has not always been the House of Evil, as it was in the books. It's always had a somewhat sinister reputation, and they seem to revel in it a bit, but Andromeda Tonks can't be the first Slytherin to marry outside her house.
I would say in the Marauder era, before Voldemort's rise, Slytherin house might have tended to keep to itself, but there were probably some Slytherins who made friends outside their house, and it wouldn't have been a huge deal. A few (like James and Sirius) would probably have heckled anyone making friends in Slytherin, but if the Slytherin seemed to be an okay person (i.e., not Snape) they'd probably have gotten over it.
But the prejudice will be there. Remember that Hagrid, decades after he was expelled (and after he's been living at Hogwarts for fifty years or so) is going around saying that "all Dark wizards come from Slytherin."
I imagine that after the first time the Chamber of Secrets was opened in the modern era (in Tom Riddle's day), Slytherin's reputation was darkened again for a long time.
There was a Sorting Hat song, I believe, that mentioned that at some times in the past, house rivalry has erupted into duels and open warfare.
Kill the Spare
There must also be cases where Slytherins have siblings in different houses. Obviously we know about Sirius and Regulus but perhaps the Slytherin girl has a brother or sister in Gryffindor which is how Remus knows her.
The most intense rivalries in the Harry era seem to emanate from the Quidditch field. Maybe your OC isn't interested in Quidditch so the rivalries don't make sense to her.
I would place her as younger than Remus- possibly Regulus' year or below.If she's thee same year as Snape then she'd be all too aware of the hatred between them all. Maybe they meet away from Hogwarts.
From the point of view of Sirius perhaps this is the point where he starts to distrust Remus. Remember he trusted Peter totally and made him the secret keeper as he thought Remus was the spy- it could explain his suspicion.
The good thing about Remus is that we know more about him then the other Marauders simply by his teaching at Hogwarts. He seems like such a gentle, good person, but he has that horrible secret, so it would be hard for him to make friends because of it.
I could see him becoming friends with someone else who was equally gentle and troubled, which might also explain a Slytherin reaching out to someone outside of her own house?
As for his friends, they seem to be kept pretty busy abusing other students, so they may not miss his absences, as long as he contributes to some of the mischief?