One possible analysis could be that their relationship in fanfic often showcases a strong bond of friendship. Bakugo's explosive personality and Kirishima's more stable and friendly nature create an interesting dynamic. Fanfic writers may explore how they support each other during battles or in their daily lives at school. For example, Kirishima might be the one to calm Bakugo down when he gets too riled up, and Bakugo could inspire Kirishima to be more confident in his abilities.
Kirishima is still alive in the manga. The story doesn't have that plot point. Usually, character deaths are significant events and are clearly shown if they occur.
Often, they feature intense action scenes as both characters are fighters. Their clashing personalities also make for interesting drama. For example, Iori's brooding nature against Leona's more stoic self.
Another characteristic is the exploration of his relationships. His friendship with Bakugo is a popular theme. Fanfics might show how they support and challenge each other. Also, his relationships with his other classmates are often explored, like how he can be a source of motivation for them.
Kirishima is from a manga/anime. His back story involves him having a sense of self - doubt initially. He admired those with strong and manly qualities. He worked hard to overcome his insecurities and develop his own strength and courage, eventually becoming a reliable hero - in - training.
A new story could be that Leona and Diana are called to a neighboring kingdom to help with a famine. Leona tries to use her sun - based powers to grow crops quickly, but it doesn't work as expected. Diana then suggests using her lunar magic to find underground water sources. They work together, with Leona digging where Diana indicates. Eventually, they find enough water to start irrigating the fields, and the famine begins to subside.