A Fault in the Set

Atsumu Miya and his twin brother, Osamu, are second years on Inarizaki's volleyball team. Just days before their birthday, Atsumu's world begins to shift after he accidentally overhears a few teammates talking in the locker room.
"Aww, Kita, you must be so worried about your best setter's health," someone teases.
"No, Atsumu, you're not the best setter, your amazing and talented brother Osamu is," another voice mocks.
Laughter fills the room.
'What...? Are they making...fun of me?'
They're probably just joking, right? They always make him the punchline. That's just how things are, how they've always been. It's normal. It has to be.
But over the next four days, the laughter doesn't feel harmless anymore. Everywhere he goes, in every passing comment and half-heard whisper, Atsumu thinks he hears the same thing: that they hate him. That he isn't good enough. The words start echoing in his head until he begins to believe them himself.
Then, on the fifth morning, he wakes up and forgets all of it.
Because it's his birthday. The day he turns eighteen.
Excitement replaces doubt. Every year, the team throws a surprise party for him and Samu. It's tradition. He carries that expectation with him all day, grinning to himself.
But when he walks into the gym, a pit forms in his stomach.
There are decorations. Music. Laughter.
Just like always.
Only this time, the party isn't for him.
It's for Osamu.
Did they... forget?