Kai doesn't like parties. Aichi, apparently, does.
Which is, of cause, not a big deal and there's nothing wrong with that, because everyone's got different opinions and Kai knows he's in general really not like Aichi and both of them exceeded this step of their relation where such little and dumb things mattered. After they've been through so many things together, have been through good times and bad times, they stopped caring about their little differences and focused on the things, both of them liked.
At least until March slowly comes to an end and someone mentions this certain thing, the thing which is Miwa's birthday, and Aichi gets kind of offended.
Well, not really offended, but then also a little bit offended, because why haven't you told me, Kai-kun, and Kai tries to ignore it, but can't, so he starts feeling guilty somehow, because, hell, how is he supposed to not feel guilty when Aichi says his name that way.
So, it ends with Aichi wanting to plan a party for Miwa and, here we are, because Kai doesn't like parties, doesn't like those social things in general, because they are awkward, but Aichi seems really excited and happy and Kai can't say really anything right now, – how could you say 'no' to this face.
They go out together, but they don't actually go out, just go out to that shop to buy some decorations for the party. Aichi tells Kai to choose a colour for the decorations, and Kai picks blue without really thinking, because he likes the colour blue and it fits well with Aichi's hair, so, yeah, why not.
But then Aichi looks disappointed again, says he thought red was Miwa's favourite colour and Kai simply looks at him, because he has known Miwa for years without ever bothering to ask him about his favourite colour, since he never cared about that and, to be honest, doesn't even care about it now.
In the end, Aichi decides blue and red fit together rather well and two different colour would probably look nice, right, Kai-kun, and, naturally, Kai agrees.
On Miwa's birthday, they decorate Card Capital together with the others and Izaki even made a cake after Aichi asked him, but they tell Kai to write something on it, because Kai is Miwa's best friend, right. So Kai ends up scribbling a 'Happy 15th Birthday, Miwa' in blue icing on the cake.
All of them are pretty excited and when Miwa enters the shop, the others shout 'Happy Birthday' and try to sing a song, but it doesn't really work out with Morikawa not really knowing the text and Kamui constantly reminding him of that fact, but Miwa doesn't seem to mind and smiles at them.
"Thanks a lot, guys, you're really awesome."
He rubs his neck, and when he spots Kai, his smile brightens up even more, at least if that's possible and he goes over and Kai stretches out his hand.
"Happy Birthday, Miwa."
Miwa shakes his hand enthusiastically.
"Thanks, Kai. Now, give me a second to look at this glorious cake and – hey, I'm finally 16 today, who even wrote that onto the cake."