Relationships
Though one day I plan to expand this out to his relationships with his family, and all of SEES for now I just want to get some notes down on The Big Two.
Shinjiro Aragaki
Prior to his (I did say there were spoilers all over the site, I hope you listened) death, and/or time entering a coma Shinji is the most important person in Akihiko's life. They have basically acted like brothers since their time in the orphanage.
They say opposites attract, and this seems to be a core part of why Akihiko and Shinjiro work out. Akihiko wears his heart on his sleeve (possible the purpose of the mysterious arm garters on his shirt) and if Shinji is trying to wear his it's under that big coat somewhere. Akihiko really annoys Shinji into accepting his duty and trying to make the world better despite the consequences of Shinji's brief loss of control.
In the Persona Q series, we get to see a more goofy side of their interactions. While the games are very exaggerated there doesn't seem to be any doubt that competition is a core part of how they relate to each other.
I mean, they have matching Personas for christssake, I don't know how much closer a pair of bros can be than having the literal Gemini twins (Castor and Pollux) as their starting Personas.
Mitsuru Kirijo
I don't think it's an especially hot take to say that after Shinji, Mitsuru is the closest person in universe to Akihiko. Besides being very close friends, even on a first name basis both characters tend to be so standoffish that getting close to anyone seems a bit of a miracle.
Thrown together over their shared ability to use Personas, Akihiko and Mitsuru also were the only members of SEES for quite awhile after losing Shinji.
More than just friends, Akihiko and Mitsuru are also both just on the same wavelength, and conduct business in very much the same way. Both are action oriented, and interrogate/investigate Strega and the Dark Hour as a team, often leaving the second years behind, or even in the dark completely on some tasks.
But are they in love though?
Despite the aforementioned great chemistry ATLUS has taken pains at several points to state they are not officially in a relationship and at least into their 20s when P4AU is set they have no plans on getting into one. In fact, P4AU is probably the point where Akihiko says most clearly that he wouldn't date someone as, uh, intense as Mitsuru.
I've never been a person who really gets into shipping, and would like to stay as far away as possible from anything that might be called a shipping war however, my opinion tends to be that with both characters being romance options (Mitsuru in all P3 games, Akihiko on the female route of P3P) it's more likely ATLUS shied away from this relationship to avoid conflicts with player love interests.
