Riki Nendou is best known for his low intelligence and frequent misunderstandings. He often displays poor manners and has significant difficulty grasping even simple concepts, regardless of how many times they are explained. This lack of awareness makes him a source of constant irritation for Kusuo Saiki, especially since Kusuo is unable to read Riki’s thoughts making him unpredictable and exhausting to interact with.

Despite his dim-witted nature and intimidating appearance, Riki is fundamentally kind-hearted and well-intentioned. He frequently goes out of his way to help others, whether it be saving someone from drowning, defending classmates like Shun Kaidou and Kineshi Hairo, or generously sharing food during the group’s Okinawa trip. He is especially affectionate toward his pet hamster, Koriki No. 2, which he received from Kusuo and occasionally brings to school.

Riki deeply values his family. Though he never met his father, he regularly visits his grave and expresses a strong desire to support his mother once even seeking employment to buy her a gift, despite his inability to keep a job due to his lack of competence.

While Riki is mostly pure-hearted, he does have a tendency toward inappropriate behavior, particularly around women. He demonstrates poor fashion sense and makes awkward or inappropriate gestures, such as suggesting unsuitable outfits for Kokomi Teruhashi. His romantic preferences often focus on superficial traits, paralleling Reita Toritsuka whose guardian spirit is, in an ironic twist, Riki’s late father.

Riki Nendou has an intimidating appearance that often leads others to mistakenly assume he is a delinquent. His most distinguishing feature is the scar over his left eye, which sets him apart from both of his parents, who otherwise share his facial features almost identically. He sports a unique hairstyle with a mohawk like cut approximately 7 cm trimmed down the center dyed bright yellow, while the rest of his hair remains black and untouched.

At PK Academy, Riki wears the standard male school uniform, but he leaves the jacket unbuttoned to reveal an orange T-shirt underneath, contributing to his rugged, casual image. Outside of school, he typically wears collared shirts. Riki also has a moustache, which he originally grew in admiration of his favorite comic book villain, Baron Cola.

Riki Nendou first appears as a freshman at PK Academy, initially portrayed as a disruptive classmate who frequently bullies Kusuo Saiki and copies his schoolwork. However, his behavior quickly changes after an incident in which he is falsely accused of theft by Takahashi. Believing that Kusuo was the one who helped clear his name, Riki develops immense admiration for him completely unaware of Kusuo’s psychic involvement and begins following him around incessantly.

From that point forward, Riki appoints himself as Kusuo’s self-declared protector. He regularly wards off other students whom he perceives as threats and constantly invites Kusuo to eat ramen after school, much to Kusuo’s dismay. Despite Kusuo’s efforts to avoid him, Riki remains one of his most persistent companions.

In a later time skip, Riki is seen as a sophomore, continuing his close association with Kusuo and Shun Kaidou. The three are frequently grouped together, with students mistakenly assuming that Kusuo is the leader of their trio due to his central position and quiet behavior.