Ao no Flag is such a great coming of age story showing you the different thinking of people who lead different lives from different environment. There's so much to talk about not just the story itself including side characters like Mami and Masuo.
To be honest, I only really did start reading it because its tagged as BL bit to be honest it's only really 5-10% BL and all or most of it is really coming of age and slice of life. I think this was the focus of the author and not as a love story, which would explain how it ended. A lot of readers complained how it ended so abruptly, but if the author meant to make a coming of age story and BL is just a background to set up the problems they need to overcome, then it's understandable.

Taichi
After primary school, Taichi and Touma has drifted apart according to Taichi. And this was because of Taichi's lack of self confidence constantly comparing himself to Taichi who's tall, handsome, can do anything, and brimming of charisma. While Taichi is short, average at things and not charismatic. This was the wedge that caused the distance between the two friends. Of course this wasn't what Touma wanted he kept on starting conversations with Taichi but Taichi couldn't keep up with his amount of friends and his being so outwardly perfect. Taichi gave up on their friendship. So if say, Futaba never came in the picture, the circumstances would have probably just have worsened. There was no problem, Taichi didn't seem like it was a relationship he wanted to maintain, in the end, he gave up on Touma.

Touma
This was not really that talked about compared to other issues. He felt like he wasn't free and couldn't do as he wanted around his brother and sister-in-law. Although they didn't say why, was it because after their parents death his brother started to support him financially and as a child he couldn't really do anything? So he had to calibrate his actions to not disappoint his brother? But of course Seiya didn't really think of Touma as a burden, it was just Touma wanting Seiya to live his life with his wife. I think the root of their problem was lack of communication resulting to lack of understanding. Despite Seiya reaching out to Touma, Touma blocked himself from it. Why? No one knows. It even took a girl who couldnt give up on him to admit ti anyone how he felt towards Taichi. And it took the whole school to talk about it for him to confess. Taichi's problem was communication, and the person who fixed it, is Futaba. When they talked in the garden after the "gay issue" was circling around school, they talked about it and I think that's when Touma resolved his communication issues.

Futaba
I think the only problem Futaba has is that she doesn't think highly of herself, this stems from how she's treated for being VERY clumsy. But she tries to understand other people and their pain. When she found out about Touma's feelings for Taichi she thought about how he must have felt when he loved Taichi first and for so long. No problem there really just her view of herself which probably have gotten better as she grew up.

Masumi
Just like Touma she has had feelings for Futaba her best friend but couldn't confess or say anything because she not only does know that Futaba likes someone else but she's also afraid of how other people will react to her being a lesbian. She does understand after talking to Akiko, Touma's sister-in-law, that you really cant do anything about other peoples response to what you are and your actions, and its more important to have no regrets. I guess she does eventually accept herself being bisexual because in the end she does marry a man who says because of how she is he tends to get jealous of both men and women around her. And it is realistic at how she was never able to confess to Futaba, not all people in real life need to confess just for closure. A lot of people were sad about her story but I think its pretty okay after thinking about it.

Mami
Mami is also a great character, cant really find anything she had trouble within herself. Her main problem was not being able find guy friends who she wouldn't have problems with. as of the moment she only has Shingo as her only unproblematic guy friend. She has no problems being friends with girls and boys, but girls start to become jealous and territorial at her when she befriends men and men only see her as a potential girlfriend material. Although, she doesn't really let that bring her down. She knows she has done nothing wrong, so she just continuous on her life. Another positive trait is, She is extremely loyal so I guess thats why her closest 2 friends don't get jealous around her because they know she'll only look at Touma. Must be great tho, being that pretty

Kensuke
Kensuke is a great example of someone who cant be friends with girls because his view is completely the opposite of Mami, his crush, (that word is so cringe). In his mind, girls are way different than men biologically, women are smaller, weaker and overall fragile for example when men can just beat each other up as a way of understanding each other, a man cant do that to a girl. And according to him there's a lot of places you cant touch a girl in while it would be fine with his bros. Girls are so different which is why he cant understand being gay. There is also that passing sentence saying he was molested multiple times which is another factor for his problems with gayness. He does know that there are individual differences between each people, for example there are men who are more feminine despite being straight and there are bros among women who are fine with punching each other out. But compared to gender differences, individual differences are way too great. Despite all this, he is fine with staying friends with Touma. He understood that they are Touma's friends, he's just having problems accepting what Touma is. You cant really hate him for that, you can still understand him. and despite his stand on being gay he still is trying to understand Touma.

Shingo
Aaaaaaand finally my favorite character. He is so insightful, he empathizes with his friends, he thinks before he talks and he really analyzes the situation before doing anything. He's also the only guy friend that Mami completely trusts that really sees her only as a friend. Even when he had a girlfriend and she complained about his friendship with Mami, he knew he was in the right and chose his friendship, because the girlfriend's accusations were baseless and jealousy driven. I also like how even when he was with Shouko, one of Mami's friends this was in ch 44 where he really shined, he corrected what they said showing how their logic was not rectifying Kensuke's perspective.