Shun Kaidou is a chuunibyou, meaning that he believes that he is very different from the usual crowd. When Kaidou first saw Saiki 's solitary personality, he became a "friend" of Saiki and began to follow him around, much to Saiki's annoyance.

Kaidou believes that he is the only person who can take on the "Dark Reunion," an evil group that is actually a fabrication in Kaidou's mind due to his chuunibyou. Despite his overconfident personality, Kaidou has expressed many attributes that say otherwise, such as hiding in the toilet and his inability to swim.