The theme of mutual understanding also frequently appears. Since they have different personalities and quirks, they have to learn about each other. Fanfictions often depict the process of them getting to know the little things about one another, like Izuku learning Tsuyu's favorite foods and Tsuyu understanding Izuku's insecurities.
Another theme is friendship and support. In many of these fanfictions, they rely on each other. Tsuyu's calm and practical nature can balance out Izuku's more impulsive side. And Izuku's determination can inspire Tsuyu. They are often shown as being there for each other through tough times, like difficult exams or dangerous battles.
One common theme is growth. Both Izuku and Tsuyu are characters who are constantly evolving in the original series, and in fanfiction, this growth is often explored further. For example, they might grow not just as heroes, but also in their relationship with each other.