Oh goodness, urm… I didn’t get into the P3 fandom much so this is going to be boring, I think.  You’re allowed to ask for someone else after I answer this if you want  :)

My otp:  Shinji and Akihiko, hands down.  You can probably convince me to add some Mitsuru in that mix.  
My most hated pairing:  Don’t really have one, since I wasn’t really heavy into that fandom.   
My unusual otp:  Is Chie or Kanji really unusual after Arena?  I certainly don’t think so  ^^ 
My crossover otp:  Again, Chie or Kanji, not really a crossover right?  I mean… 
My brotp/friendship otp:  Ikinda feel like Mitsuru and Akihiko are my friendship otp that ends up sleeping together a couple of times.  I don’t think they mean to, but it just happens.  It’s kinda like my Yosuke and Chie otp, I see them sleeping together, maybe a couple of times, but they don’t become a couple.  And they’re cool with that.  I think Mitsuru will be around Akihiko in some way, shape, or form for a long, long time, same with Yosuke and Chie.  I guess Mitsuru and Akihiko are sort of a more mature Yosuke and Chie for me.  
Character headcanon:  I think Akihiko really cared for Shinji, was probably in love and all that jazz, but it was sort of a doomed relationship from the start (considering the whole S.E.E.S thing and such).  They weren’t really together but the feelings were there, they’ve messed around but probably never actually said the words “I love you,” which is something Akihiko regrets, to this day.  Akihiko will always have Mitsuru, but I don’t really see them as characters that will be in a relationship.  I don’t see them as being characters who can settle down like that, but they’ll always be there for each other, maybe even cross the sexual line, but having a family and kids and things like that?  No.  And that’s not a bad thing, they’re content with that.  

I also think that Akihiko sees himself in Kanji, is maybe even a bit jealous, because you know, the whole theme of P4 is “facing yourself and accepting yourself,” and Kanji has done that at such a young age.  And Kanji’s feelings for Naoto are pretty loud and clear, and that’s something Akihiko never did with Shinji.  There’s hope for Kanji to be with who he wants, but Akihiko lost that chance.     
A Gif that shows how I feel about the character:  I have like no gifs (I know so weird right?!) but I really liked Akihiko :)