Kurumi is Kusuo's carefree mother and a major role in Kusuo's development. She is the main reason why he doesn't use his powers for evil and turn to the 'dark side.' Kurumi is ecstatic whenever she see Kusuo making friends. Kusuo respects his mother and sees her as a kind, but too trusting person, but is a little frightened of her when she snaps at times.
Kuniharu is Kusuo's father. He is more open to taking advantage of Kusuo's powers. Kusuo loves his wife and sons a great deal and is excited to see Kusuo's friend, though he sometimes enjoys seeing Kusuo in his misfortune as payback for his disrespect towards him. Kusuo pays very little respect to him ever since he was a little baby. He usually refuses to help his father, whom he thinks would become too dependent on his powers, though he can be bribed with coffee jelly. After seeing how hard he works at his office, even though he made mistakes, Kusuo gain a little bit respect for his father, but still at times find him annoying.
Kusuke is Kusuo's competitive older brother. They have been competing with each other ever since they were little. However, Kusuo with his psi powers would always win against the genius Kusuke. Kusuo always thought that his brother hated him for always winning, but Kusuke is actually obsessed with Kusuo. He enjoys competing with his little brother even though he knows he will lose, showing his masochist side. In twenty years into the future where Kusuo dies early, Kusuke is happy to see him since his life was boring without him. Kusuo does not like his brother very much, finding him very annoying and wants very little to do with him. Kusuke was the same person who created the limiter on Kusuo head to hold back his power.
Chiyo is one of Kusuo's classmate who fell in love with Kusuo. She fantasized about him and tried to get closer. However, Kusuo, not interested in romance, easily foils her plan every time and was able to make her move on and start dating Shinichi. After three months, Chiyo once again started to think of Kusuo as her relationship with Shinichi was souring. Kusuo tried to get them back together which worked at first but failed in the end due to his bad habits. Luckily, Kusuo eventually was able to get her to fall in love with Kaido at the beach by increasing the strength of Kaido punch to keep the beach punk away from Chiyo and Kokomi.
Kokomi is one of Kusuo's classmate who noticed Kusuo walking alone in town. She tries to "improve" his summer by chatting with him, but Kusuo barely acknowledged her. She assumed he saw her as an illusion and that he was too infatuated to see her. She tried to get a reaction out of him, but failed as Kusuo is not interested in romance and believe she would draw too much attention. After Kusuo teleported away, Kokomi ended up thinking that he was an illusion and she was the one in love with him due to her belief on love, much to his dismay. Kokomi is persistent on getting Kusuo to acknowledge her that she'd go that far as to eat gross ramen with him and his friends , something that did impressed him. On Valentine's Day, Kokomi brought chocolates for Kusuo, but wasn't able to give it to him in fear that he’ll get beaten up by her fans or would start World War III, leading her to throws it away. Kusuo ends up catching it anyway. During the school trip, Kusuo saves her from a bear after she was teleported to an island by mistake. She assumes it was a dream and took advantage of it by holding his arm, take her to a castle, and makes "Kusuo" acknowledge her.  He tries very hard to make her hate him, but every attempt was unsuccessful. During their 'date' Kusuo made terrible habits so Kokomi could hate him. The plan almost work until her obsessive brother showed up, leading Saiki to take her hand and hide, which made Kokomi embarrassed and regrew her interested in Saiki, believing she was into 'aggressive' men.
Riki is one of Kusuo's classmate who's mind he cannot read. At first he was shocked, but came to believe the reason was that he is an idiot. Riki sees Kusuo as his best friend after helping him prove his innocence, often asking Kusuo to eat ramen and calls him partner much to Kusuo's dismay. Kusuo dislike Riki not only because he's an idiot, but because he could not read his mind, leading him to secretly fear him and believe one day he could sneak up on him and kill him. Despite his annoyance of Riki, Kusuo at time helps Riki with some of his problems and at times admits he isn't a bad person, even slowly getting somewhat use to him. But at time find him annoying.
Shun is one of Kusuo's classmate who suffers from chuunibyou. Shun goes to great lengths just to befriend Kusuo, believing he's lonely just like him. He went around Kusuo's neighborhood to find the 'trickster' in 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.  Despite finding Shun troublesome at time, Shun is possibility the least dislike person Kusuo knows since he is more willingly to help him and at what point called him a friend when he was being tricked several times. Kusuo, still believe he's a crybaby, find him brave when protecting a girl from a snake attack and protecting Chiyo from beach punks. But like any other, Kusuo still find Shun annoying with his chuunibyou, but soon got used to him.
Hairo is the class representative of Kusuo's class. He is a hot-blooded young man who is reliable and gets along well with everyone even towards the outcasts like Kusuo and his circle of friends. Despite finding Kineshi overbearing at time, Kusuo does respect his strong will and how well liked he is in school. But still find him annoying by his wild nature, but somewhat got used to him.
Imu is a junior classmate who was used by Kusuo in order to make Kokomi leave him by making it look like they were dating. However, when he tried to go along with it, it backfired and Imu started liking him for real. She later realized that it was Kokomi that she likes.
Mikoto is a Sooth Sayer and one of the few who knows about Kusuo's powers. After witnessing his powers, she started stalking him and tried to get close to him. She is a member of Occult Club with Kusuo, Chiyo, Reita and Arisu.
Reita is a Spirit Medium and one of the few who knows about Kusuo's powers. He found out from a spirit who saw him use his ability. Kusuo ranks Reita as his number one most hated guy for finding him annoying and being a pervert. Kusuo usually messes with him like when they were dividing pairs for the test of courage and Reita wanted to be paired up with Arisu but he paired Reita with Chiyo. Though Saiki does feel bad for Reita having to deal with annoying spirits all day, especially Nendou's dad.
An old acquaintance of Saiki's and one the only people who knows he's psychic.