Alrighto, decided on an interpretation of Adult!alterAtsuya.
So after S2/IE2, alter!Atsuya went to sleep deep within Shirou. He decided to do this bc he believed it was the best course of action for Shirou to be happy. From then on, Atsuya existed when Shirou didn't.
Years passed, then Shirou and Atsuya had a talk and decided to coexist, building separate lives while supporting each other and all. Maybe Atsuya was feeling a bit lonely and decided to make his existence known to Shirou or smth.
Shirou is the one who played as part of the national team. Shirou is the one who is both an awesome defender and a striker. Atsuya acknowledges his brother's soccer talent and he's not any worse himself.
Also, I'm inclined to consider that Shirou acknowledges that alter!Atsuya is a completely different entity from his actual deceased brother. Alter!Atsuya has always known ofc but young Shirou couldn't grapple with that fact. As young adults, Shirou acknowledges alter!Atsuya as an independent existence. Thus, Atsuya decides to change his name to some Japanese character related to heat/passion but read as "Atsu." He likes this bc from what Shirou told him, he would have gotten along swimmingly with the boy had he survived. Atsu and Atsuya are similar in some aspects, they are passionate about their interests and protective of their loved ones. As they get older, Atsu becomes more different than Shirou's memories of his brother.
(NGL I'm using AreOri Atsuya as a reference bc I can't pull this stuff outta nowhere. I will sprinkle some differences here and there, particularly on his characterization as an adult which is nowhere in canon)
So for my fic, I want to make a callback to the Aliea Arc. Atsu's role there was really specific and relevant to the struggles I want to portray with Shirou. Basically, Atsu as a little devil that acts like Shirou's guardian angel. He knows what Shirou needs but like back then, any advice he gives to Shirou will go in one ear and out the other. So Atsu is like the meme "oh shit, here we go again" and devises a plan to get Shirou out of his rut.
Enter, Someoka Ryuugo.
And that's all I got for now!
#InazumaEleven #FubukiShirou #AlterAtsuya #MaryWrites #SomeFubu