Akihiko had always been fine with how gloomy Shinji was. He had been gloomy ever since they had first met, and the cloud of negativity that followed him like a shadow only seemed to grow along with Shinji himself. By the time they were readying to enter high school, Akihiko himself was about the only thing left that could bring a smile to Shinji's face.
Shinji wasn't good at making friends. Girls didn't flock to him like they did Akihiko, and boys didn't approach him to ask for help with their studies or athletics. Despite the crowd always vying for Akihiko's attention though, he wanted it from no one but Shinji.
Ignoring the strange desperation of his peers to become close to him, Akihiko instead worked to close himself off, stay close to Shinji, keep Shinji happy and make him feel wanted. None of the people who wanted Akihiko around allowed him to bring Shinji, so Akihiko refused to so much as give them the time of day. They were happy though, the two of them. As happy as they could ever be, in any case. Shinji would cook for Akihiko every evening while Akihiko helped Shinji study every night.
It wasn't until Akihiko realized that he only ever felt truly fulfilled when he saw Shinji smile that there might be something a bit … different about their friendship.
And that was how Akihiko knew.