In 'saiki kusuo x brother fanfiction', the plot could range from comical to heart - warming. Maybe there are stories where they team up to deal with Saiki's psychic - related problems or just have normal brotherly adventures. The fanfiction gives an opportunity to expand on the characters beyond what is shown in the original work, adding new layers to their relationship.
Well, in fanfiction about Saiki Kusuo and Teruhashi, often their relationship is explored in really interesting ways. Sometimes it shows Teruhashi's attempts to get closer to Saiki in a more comical way compared to the original series. And Saiki, with his unique powers, might react in different and unexpected manners.
Another theme is the idea of Teruhashi trying to break through Saiki's emotional barriers. Since Saiki is often aloof, Teruhashi's attempts to get closer to him form a major part of many fanfics. It can be really entertaining to read how she tries different strategies to get his attention.
Friendship within the family is also a common theme. Despite being brothers, they might have a friendship - like bond that is explored. For instance, they could have inside jokes and shared hobbies that are unique to them. Also, the theme of protection can be popular. Saiki might use his powers to protect his brother from various threats, or vice versa.
One popular theme is the 'unlikely couple' aspect. Given Saiki's introverted nature and Teruhashi's popularity, the idea of them getting together is an interesting concept. Another theme is the comical misunderstandings. Since Saiki has psychic powers and Teruhashi is always trying to be perfect, there are a lot of opportunities for misunderstandings that lead to humorous situations.