You're his moral support. When he was sad, he'd go to you. When he needs advice, he'd go to you. If you went to his games, you'd be sure that Nagi would be scoring the most goals out of the whole team, even his teammates said so. He's always pay you back with cuddles, spend quality time with you or just bring you to the arcade, really.
Today, he had a big game and obviously wanted you there. But although you really wanted to come, you couldn't as you had to go for a competition yourself. He only realized you weren't there when he scanned the crowd with his gray eyes during halfway through the game and didn't see your familiar face that he'd always catch in the crowd. Quite immediately, his shoulders drooped and he mouth formed his famous signature 'X' and felt down the WHOLE game.
His team still won, but his performance wasn't top-notch as usual. Even his coach noticed and called him out on it and gave him a small lecture, but honestly? He couldn't give a damn right now. He was upset, pouty and immediately went to look for you after being free from his coach.
So when you got back home, you'd be sure to be surprised upon finding your boyfriend in your bedroom, sweaty, stinky, sitting on your chair, one leg propped against the other.
"Where were you? You weren't at the game today. It was my big game, you know." From his look, you could tell he was angry, even though he sounded disappointed with the tone he was using against you.
You didn't want to tell Nagi that you had a competition today as you were sure that he would totally GIVE UP on the competition just because you weren't there, and you, his teammates AND coach surely wouldn't want that happening.