A Strong BondBefore I start looking at Akihiko and Mitsuru's relationship, I want to state that I am fully aware neither of them have romantic feelings for each other. Yes, I like the idea of their relationship being more and I do think the potential is there. However, I also know they are both available for the protagonist to date and therefore cannot be interested in each other. My intentions on this page are to analyze their actual relationship in the game and what they think of each other. If you are interested in my thoughts about their potential or reasons why it'd be complicated, please view those pages respectively. The point of this page is to basically draw some conclusions based on the scenes I described on the previous page.
On a surface level, Akihiko and Mitsuru seem to get along well. They are both mature, level-headed and take most things pretty seriously. Further, they both appear to respect and value the other's opinion when making decisions. Their interactions on screen can pretty much be considered "professional" despite knowing each other for a few years now. This may seem weird at first, but if you consider their interactions with others in general, it isn't very surprising. They are both characters who keep other people at an arm's length and don't let others see how they are feeling. Despite this, I would still argue the two have a strong bond because of the times they do show their feelings. Times that come to mind are any time when Mitsuru scolds Akihiko, when Akihiko yells her name during the third full moon battle and when the group witnesses the door to Akihiko's past. Their professional nature drops and there is a soft familiarity...for lack of a better word. It's hard to pinpoint exactly what it is about their interactions that makes me feel this way, but you can just tell they've known each other for a while. Do I think they are super close and know everything about each other? No. But they were the only ones with Persona for a long time and likely formed a bond by working together and putting their lives on the line in secret. In a sense, they kind of "grew up" together by being forced to mature faster than others their age. They shared something no one else knew about (other than Shinjiro and the Kirijo Group) and were forced to live different lives because of it. People naturally gravitate towards others who understand their situation.
I would also argue a lot of their interactions must occur off-screen. Akihiko seems to know a lot about Mitsuru's past and what goes on after the death of her father based on comments he makes. Therefore, I'd assume Mitsuru confides in him at least a little. She certainly doesn't tell him everything or rely on him because of her personality, but I think it's as close as she lets herself be to someone initially.
Akihiko's feelings for Mitsuru are definitely made more obvious than the other way around. He straight out says Mitsuru is one of the three people he cherishes and therefore wants to protect by all means. Akihiko is also openly worried about Mitsuru when she is injured during the third full moon as well as after her father dies. In fact, he talks of nothing else until she is able to move forward with her grief. Akihiko also seems to be slightly intimidated by her. He knows not to go against her warnings to stay out of Tartarus until he is healed (though he considers disguising himself in order to try). Akihiko is also painfully aware of Mitsuru's "executions," as he fearfully explains to Junpei and Ryoji what she will do if they get caught in the hot spring with the girls. Lastly, he comments on how Ryoji has guts in order to ask Mitsuru out when he gets word of him doing so. Therefore, the idea of asking Mitsuru out must seem pretty intimidating to him as well!
It's much harder to pinpoint Mitsuru's feelings for Akihiko because she rarely talks about them. She seems to treat him like an excitable child in the beginning but does it with a sense of fondness. It feels almost like a familiar pattern of interaction. I think Mitsuru is genuinely concerned for his safety though, based on how she chastises him and is irritated when he doesn't call her if he's going to be late. When the protagonist asks her if she is in love with Akihiko, she says their relationship isn't like that. However, she also can't pick a word to describe it; Both "friend" and "teammate" are insufficient. I think Mitsuru struggles to define the nature of their relationship because she has never stopped to consider it. Based on how she assumes Yukari no longer has a reason to be friends with her without a common "enemy," Mitsuru likely sees the SEES members as just people trying to reach the same goal. However, Akihiko has been there with her for a long time. He cares about her and they've been through a lot together. Such a relationship is not familiar to her and therefore it'd make sense for her not to know what to call him during this time. I do truly wonder what she would say to the same question at the end of The Answer...
All together, it would seem Akihiko's feelings for Mitsuru are stronger than hers for him. I don't know if that's simply because Akihiko is more in tune with his own feelings or if he really does just feel more strongly for her. Akihiko had close relationships with Miki and Shinjiro in the past but Mitsuru was an only child and seems to have been raised in a strict, professional household. She may not know what it's like to feel close to someone and/or express her feelings for others. Again, it would be interesting to see more of their interactions after the events of The Answer when she's grown tremendously as a person. Generally speaking though, I would just consider them to be good friends who have shared powerful experiences.