Another theme is the exploration of their differences. Shigaraki represents chaos and destruction while Deku represents hope and heroism. Fanfics usually explore how these two opposing forces interact and influence each other. For example, in some fanfics, they start to understand each other's perspectives better as they spend more time together.
To write their relationship realistically, you need to consider their personalities. Deku is kind - hearted and determined, while Shigaraki is more cynical and vengeful. So their interactions would be full of tension. For example, if they have an argument, Deku might try to reason with Shigaraki using his ideals, but Shigaraki would likely mock or reject them.
One interesting plot could be that Deku and Shigaraki, as brothers, are on opposite sides at first. But then they discover a common enemy that forces them to work together. This would involve a lot of character development as they have to overcome their differences. For example, Deku's belief in heroism and Shigaraki's hatred for the hero society.
One great 'Shigaraki x Deku' fanfic is 'Unlikely Bonds' which explores a unique friendship-turned-something -more between them. It delves deep into their complex characters and how they might interact in different scenarios outside of the main plotline.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
Some characters might try to understand Deku's motives. Kirishima, who was close to Bakugou, would be angry at first but might also try to find out if there were any extenuating circumstances. However, many would be afraid of Deku, seeing him as a threat now that he has killed one of their own.