Year 1
They didn't have much of a choice really. Seamus wasn't exactly prepared to make Neville Longbottom his best friend for seven years so Dean had to be it. It wasn’t that there was anything wrong with Neville as a roommate or classmate, but whenever Seamus tried to talk to the boy he seemed to panic. Sure, Seamus was loud, but Neville didn’t have to flinch everytime the Irish boy spoke up. Neville also didn’t seem to understand him half the time. Sheltered as he was, Neville seemed a little scared of Seamus extroversion. Dean on the other hand just laughed at it, Dean enjoyed his brutish charms. Dean seemed interesting enough, he was funny enough.
No scratch that the boy was hilarious. Seamus hadn't laughed so much in his entire life!
One could argue that Dean didn't actually try to be funny, Seamus just found it hilarious that he knew so little about magic.
"Have you seriously never even seen a garden gnome?" Seamus lay on his side on his new bed, doubled over in laughter staring at the boy who had the bed next to him. The boy named Dean Thomas was not amused.
"I told you, I'm a magl-ma-"
"muggle?" Seamus piped in.
"Yes that…" Dean frowned, he didn't seem to like being put under a label which Seamus could understand.
"since I'm muggleborn I grew up doing muggle things, like play football"
Seamus eyebrows rose, what -ball? Did they have foot shaped balls or something that they played with and what did they do with these footballs?
Dean seemed to notice his confusion and told him it was a sport.
Seamus then remembered that his father used to watch something like that on the tellie and it seemed fun. He just hadn't paid it much attention before, since he preferred Quidditch. Maybe if his mother had kept her secrets longer he too would’ve grown up playing this football. It sparked an interest in him and he moved over to Dean’s bed to learn more. Gradually he started imagining this alternate Seamus who didn’t discover his wizard heritage until much later and who played football with his dad instead of Quidditch with his mum. Dean was simply excited that he knew something Seamus didn’t and was all too happy to get to share about his hobbies.
Seamus was so enamoured with his new friend he didn’t even register that he shared a dorm with a famous person. Not until Dean asked him what the deal was with Harry. Unlike Seamus, Dean had noticed the extra attention their new roommate received. It took Seamus a moment to remember, he wasn’t that involved with the story anyway, but he said it was something that happened to him as a baby. His mother had said so, but he wasn’t sure. So naturally, uninterested as he was, he dove straight back into comparing football and quidditch.
They spent the first night together in the castle teaching each other about the sports of their different childhoods and somewhere along the way they became inseparable.
"But have you seriously never seen a garden gnome?" Seamus said, with the same intonation as he asked Nearly headless Nick how he could be nearly headless.
Dean sighed and put his book down; he was getting a bit tired of having to explain this time and time again.
"No Seamus I haven't, only the ones that stands still in your garden because you put them there"
"Ah, yes they do that sometimes… tries to trick ya!"
"No, I mean a plastic one"
When Seamus put his head to one side and furrowed his brow, it became apparent he didn't get it, so Dean gave up.
"You know what, never mind!"
"You're right!"
Dean's eyebrows shot high up. Did Seamus seriously agree to drop it? No follow up questions or anything?
"I'll just show you one next summer when you come visit me!" the Irish boy exclaimed and dropped the subject as he flopped down next to Dean. Leaning on him as he too started reading.
Dean didn’t continue reading, he just sat and stared at Seamus.
Honestly, Dean wasn't even aware that he was going to visit Seamus' place, it was certainly news to him but as it was said he accepted it. Gladly. He was actually quite excited. Coming from a muggle home he’d never seen a wizarding home, and he was curious. Granted Seamus was ‘half-and-half’ as he liked to say, but clearly he was closer to the wizarding community than Dean was.
Dean started to call him milkshake, not just because he was white of course, but every time something exploded around Seamus the table tended to shake. And to be fair things exploded around Seamus a lot. They’d only had one day’s classes and his eyebrows were already completely singed off. And to Dean’s defense Seamus was the one who called himself half-and-half first.
Seamus simply laughed and in retaliation started to call Dean Instant Cadbury.
And then that was it. And so they were officially best friends.
They spent their second night together in the castle discussing and planning what they would do next summer at Seamus's place.
They sure were different, but that meant they had a lot to teach each other, and for Dean it was nice to have someone who knew about this new world. At least more than he did, it was an added bonus that Seamus understood most of his muggle references due to having a muggle father. If one more “pureblood” gave Dean a funny look as he asked for a normal pencil, he’d scream. How was he supposed to know they only used quills?
Dean needed someone who could guide him when he was confused.
Unfortunately a lot of the time Seamus answers just made him more confused, which was only natural coming from an eleven year old who didn’t have a full grasp on life yet. But the obvious things, who were obvious for everyone except the muggleborns, Seamus explained without judgment. Seamus in turn benefited from hanging out with someone with a cool head, who could pull him down to earth and keep him from setting too many things on fire.
It only took a few weeks before everyone knew that Seamus Finnigan and Dean Thomas were now best friends and inseparable. That was mostly Seamus's fault really, loud as he was.