Seamus is a bit like Ron's backup friend. The bloke who's always there for a good laugh and a bit of butterbeer. Someone you could ramble to forever about Quidditch, even if he supported all the rubbish teams.
They always spent a lot of time together in the even years. 2nd, when he and Harry needed a break from each other, crazy with worry over Hermione. 4th, when he got into stupid fights with the both of them. And now 6th. Where he's still getting into stupid fights with the both of them.
That doesn't happen with Seamus. Other than the whole Voldemort issue last year, they've never really fought over anything more serious than Qudditch or the answers to last night's Potions homework or dark vs milk chocolate. And Dean's dating Ginny (he still can't get over that) and spending less time with his best friend, so Seamus's as free as he is.
They've gone to Hogsmeade a bunch too (hence the chocolate debate). Seamus was great company. They'd even gone to Madam Puddifoot's, and made daft jokes no one else would ever find funny and ordered the most disgustingly sappy things on the menu. To top it off, Seamus'd bought him a bouquet of weeds, which he still had on his desk, and made him snicker every time it caught his eye. And it didn't feel weird having him pay for it like it did whenever Harry got him a gift. Ron'd shelled out for their sad excuse of a meal after all
And in the common room, he'd sometimes lean in real close when they were checking each other's half-assed essays, which felt odd but nice. Seamus probably needed glasses and was too chicken to wear them. Or he'd fall asleep on his shoulder, and Ron'd be afraid to move in case his head jolted against something else and he lost his memory forever and moved to Australia to become a news anchor (Hermione had shown him a bit too many soap operas and he's forever traumatized). Or at dinner, or breakfast, or any meal really, their fingers would brush together under the table, completely on accident, and sometimes Seamus would forget to pull away. He was forgetful like that quite often, which completely explained his OWL scores. And added to Ron's completely rational fear of brain damage.
But despite his presumably severe mental and visual handicaps, Seamus was an absolutely brilliant friend. He'd said as much to Ginny who just smiled mischeviously in response. He hated that smile.
"I bet he is." When he pestered her to tell him what she meant, she just laughed harder. Crazy bird.
He and Harry had made up by now. Had to, for Quidditch. Ron'd been terrified that he'd be a shit keeper, but Seamus'd told him he couldn't be worse than any of the Cannon's current lineup, and their bickering had helped. Seamus was good at dealing with Ron's nerves in a way Harry didn't have the time for. Was too important for.
Harry would always be his first, and best, friend. The friend he had to worthy of. Seamus was just... different. Ron wondered if friends could be equally important, but fill completely different roles in someone's life. He wouldn't know. He only had three.
Seamus showed up for every game, and most of the practices, even though he hadn't made the team. And it made him happy that someone cared enough to watch, even if it was just because Seamus was one of the biggest Quidditch fans he knew, in a school full of Quidditch fans. Normally, knowing someone was there for him would make him fuck up harder. But this was Seamus, who'd take the piss out of him anyway, no matter what he did. So he relaxed, and realized he was a bloody good keeper.
And after every game, win or lose, after changing and catching up the rest of the team, he caught up with Seamus and listened to (and argued about) his post-game commentary. When there was an after party, after getting drunk and listening to Harry laugh more with Katie Bell than he had back when it seemed like their trio and his family made up the entire world, it was Seamus he curled up next to. And Seamus seemed to get a lot more forgetful about where his hands ended up when they were drunk. Hermione had told him before that alcohol impaired brain function and he guessed that doubled when it came to someone like Seamus.
Hermione's still his friend. Probably will be forever, quarreling when they're 90 and hitting each other with their canes. Something told him once that they could be more, that he should want more, but he didn't. Not really. Maybe they could've been, in another life where he needs her to feel confident in himself, where he never realizes his potential, and there's a lot more potion shenanigans. But this Ron isn't that Ron. He tells her about Seamus, because talking about him makes Ron happy, deep down in his bones in a way he's never felt before, and Quidditch, which makes something in him feel worthwhile that's never been satisfied, and how he dreams of having a future beyond working in some dead end ministry job. And she tells him about arithmancy, and her addiction to trying to solve things that can't be solved, and the pressure she's always felt, how sometimes she gets so depressed all she can do is work, how she feels like she'll never be loved. And he's not going to be the one to love her, not the way she wants, but he loves her all the same. And maybe in the long run that's better for the both of them, so headstrong and bitter, longing for something the other couldn't give.
When he thought about his plus one for Slughorn's Christmas party, Seamus was the first person to come to mind. He certainly wasn't going to ask someone he barely knew. He'd seen how that ended up back at the Yule Ball.
Hermione and Neville were doing this weird half-courtship routine, too insecure to not pretend that the other wasn't interested, and were going as "friends". Harry was actually going as friends with Luna Lovegood, which Ron hadn't seen coming. He was rubbish at divination though, so that wasn't saying much. Ginny and Dean were somehow still a thing (which crossed them out) but they'd moved their snogging from corridors to closets, which he greatly approved of.
So if even Harry was going with a friend, why shouldn't he? It'd be heaps more fun than inviting anyone else. So he did, on a lazy Saturday in the common room, when they were struggling to look like they were paying attention to Hermione and Neville's discussion on sentient fungi - which doubled as a badly hidden flirting attempt - and failing miserably. And when Seamus said yes, his chest grew warm and he knew he'd never want to ask anyone else. He quickly shooed that thought away. Maybe Seamus's condition was contagious.
And at the party, when he leaned onto Ron like always, under the glow of the fairy lights and buzz of arse licking conversation, the lyrics of his mother's favorite Celestina Warbeck song came to mind. Luna looked at him then, pausing her lengthy explanation of the empire-crushing effects of gum disease, and she knew. He wished she'd tell him what it was. That she, or anyone really, would let him understand why he so badly wanted to uncover the mysteries of this snarky Irish boy with no sense of personal space.