Shinjiro AragakiAkihiko and Shinjiro grew up together in the same orphanage. They quickly became good friends and eventually their relationship evolved into being practically brothers. The two are almost always fighting because they both share one personality trait in common: stubbornness. As they got older, the fights became less physical and more verbal, though! After Miki's death, however, Shinjiro takes it upon himself to be more like a caretaker for Akihiko than a brother. He matures quickly and pushes his own feelings of grief aside in order to be able to support Akihiko. Shinjiro even follows Akihiko to SEES even though his potential was not as strong because he didn't want to leave him alone. Akihiko says in retrospect that he had concerns, but I'm sure he was glad to have his friend with him. Though their past history is not explained in great detail, Akihiko really looks up to Shinjiro based on the interactions we do see. Akihiko is very persistent in asking Shinjiro to re-join SEES and is practically ecstatic when he does. If the protagonist talks to him in Tartarus with Shinjiro also in the party, he says, "I can't believe I can fight alongside him again...I'm glad I was persistent." This quote really made me realize not only how much Akihiko missed his friend, but also how much he looks up to him as well. In the fifth character drama CD, Akihiko admits even though Shinjiro can be harsh and overbearing at times, he always needs his words when they are given. He greatly respects Shinjiro's advice and wisdom...although not always in the moment. Shinjiro is the only person who can get away with confronting Akihiko and his behavior for that reason.
Additionally, it is quite clear how much Akihiko cares about Shinjiro. Part of the reason Akihiko is so persistent in asking him to re-join SEES is because he worries about his lifestyle on the streets. He worries because he knows how Shinjiro is and that he is suffering alone. In the same drama CD previously mentioned, Akihiko (who is already annoyed from being scolded by Shinjiro earlier in the evening) chastises him about wandering the streets late at night and tells him it's about time he changes his unconventional lifestyle! Yes, Akihiko was ticked at being scolded. But his irritation only showcased the concerns he has had for a while. Further, when Akihiko reads a note that says something along the lines of "I can't take this anymore. Don't look for me," he immediately assumes the note is from Shinjiro and frantically runs around Iwatodai trying to find him. Why? Because he doesn't want Shinjiro to leave again--especially not because of something he said. He wants Shinjiro to be back at the dorm where he knows he is doing well. He wants to spend time with him. Akihiko admits after Shinjiro left the dorm, he would make up excuses to go see him. He slugs Shinjiro across the face when he finds out about the Persona suppressants because he's so mad at him for endangering his life. He is also completely devastated when Shinjiro is killed/falls into a coma. The scene at Shinjiro's alter/in his hospital room is one of two in which Akihiko completely breaks down and the only scene in which he cries. He begs Shinjiro not to leave him behind. There is absolutely no question about how much Akihiko cares for him.
Shinjiro's importance makes his leaving SEES and emotionally closing off from Akihiko all the more painful. The story does not delve into Akihiko's feelings regarding this very much, but I can only imagine how hard it was for him. He may have made up excuses to go see him frequently and obviously the two stayed in touch, but it wasn't the same. Shinjiro had always been there to support Akihiko. Suddenly, Shinjiro withdrew into himself and was no longer emotionally available to him in the same way. Emotionally losing his best friend and having his primary support system pulled away from him had to have been difficult. After Shinjiro's death/coma status, Akihiko does briefly reference his hurt feelings:
"Say something, will ya...? Why are you always like that? You're so stubborn. Put yourself in my shoes for a change..."Akihiko knows Shinjiro better than anyone else. He knew Shinjiro pulled away out of guilt and gave up his old life for the same reason. He knew that, but it didn't make his best friend leaving any easier. Shinjiro was hurting, but Akihiko was hurting as well. Even so, Akihiko refused to give up. He cared too much about Shinjiro to let him shoulder guilt alone. His happiness over Shinjiro rejoining SEES was unfortunately short-lived because of his death/falling into a coma. However, Akihiko was able to not lose himself in grief because of his respect for him. He knew Shinjiro wouldn't want him wallowing when he had more important things to do. Shinjiro had always, always looked out for him in his own way and he was confident that wherever Shinjiro was, he would always be watching. Shinjiro's death/coma was only yet another example of the major impact he had on Akihiko's life.