You might need to look for specific story missions or events where the character can be shirtless. It could be part of a certain storyline or a specific interaction.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
I'm not sure specifically about 'aang shirtless fanfiction'. Fanfiction varies widely across different platforms. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They might have some related works if it exists within the community's creative output.
I'm not sure specifically as there could be many 'shirtless kid' stories. It might be about a kid who was shirtless in a particular situation like at the beach having fun, or perhaps during a hot day in a neighborhood playing outside without a shirt.