Kaidou Shun

Shun Kaidō (海藤 瞬 Kaidō Shun) is classified by his classmates as a chūnibyō. Calls himself The Jet-black Wings and thinks that the world is ruled by an evil organization called Dark Reunion. For showing off out of the blue, everyone cast him as an odd man. He may be showing off but when push comes to shove he have a good sense of justice like endangering himself for others.

Background
Shun used to be a shy person who doesn't have friends and tried to change like going to arcades, changing his hair color until he became a chuunibyou. In the end however he never gained friends and instead developed split-personality. He sees some similarities with Kusuo thats why he wants to befriend him  Shun has chuunibyou, meaning that he believes that he is a superhero. When Kaido 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.

Saiki Kusuo
Shun went on great lengths just to befriend Kusuo. He went on Kusuo's neighbor just to find the alien on his drawing which was actually Kusuo. In the end, Kusuo had no other choice but to show up with the face of the person in his drawing. Shun then started following him around, much to his annoyance.

Nendou Riki
Shun and Riki at first argued but later on got friendly with each other.

Kuboyasu Aren
Shun was the second person to know about Aren's secret of being a former delinquent. He was scared of Aren but later on went friendly with each other after Aren saved him and Kusuo from delinquents.