Ultimate Supreme Leader
Kokichi describes himself as an evil supreme leader, and often claims that this title is the reason behind many of his questionable actions as if he is expected to fulfill his role. He is an elusive and manipulative person who unabashedly tells lies and says whatever he's thinking. He is also strangely straightforward about his supposed evil nature, openly claiming to be a leader of a large secret society, admitting to being a liar, and saying he has done nothing but bad things. Among other things, his organization supposedly has covert agents in every country, control over all the mafias in the world, torture as a form of punishment, and Kokichi's defeated opponents apparently will get sent to Siberia. However, none of his claims have been confirmed, and the other students think he's either lying or delusional.
According to Kazutaka Kodaka, Kokichi is a trickster character in the truest sense of the word. He is very mischievous, and he enjoys pranks and teasing others, sometimes in quite mean-spirited ways. He appeared to have a strong need for attention, as he got irritated if people talked over him or didn't react to his antics the way he wanted them to. He also appeared quite arrogant, clearly considering himself very important part of the class trials, looking down on some of the other students, often wishing to lead the debates and once claiming that the class trials won't be good if the others won't allow him to speak. Because of his behavior, he is viewed as an antagonistic figure among the group, but this doesn't seem to stop him from being able to manipulate the class trials anyway. He is often portrayed smiling slyly and laughing mischievously, though he may look very harmless and innocent at first glance. He often behaves calmly and coolly in a way to torture the others, at times turning very ominous to the point of having very unsettling, downright demonic looking expressions. Sometimes, he can speak very sweetly, even flirty and claiming he loves the other person very much, while during other moments he can speak in a very crude and nasty manner (especially towards Miu) and use insults which vary from quite harmless to very mean-spirited. He can also try to use his looks to his advantage and sheds crocodile tears by crying loudly like a scared and threatened child, telling the others they're being really mean, but this is always overly dramatic and obviously fake. There are also moments when he appears scared, tearfully mentioning that he is afraid of the killing game and doesn't want to die, and often crying loudly soon after someone's death...but it's unclear if he is being genuine or just trying to earn sympathy with his looks. There are also moments when he suddenly falls asleep and snores comically, angering the others.
Fitting his appearance, Kokichi often acts notably child-like, with at least some of it appearing genuine while part of it also could be just his attempt to use his looks to his advantage. He is sometimes shown to have child-like excitement and innocence, as he gets very excited about "cool" new things like the Exisals and the morphing Trial Grounds marvels the school in a child-like excitement and shouts "yodelay-hee-hoooooo" while the students are inside the echoing tunnel. Kaede considered him exhausting and like a child who wants attention with bad behavior. He often acts playful and seemingly appears more ignorant than malicious during the first time he meets K1-B0. He is also notably interested in games and often has a strong desire to be a winner and play them seriously while also having fun along with them, with one of his principles being "to have fun" with everything he does. He enjoys hard difficulty, claiming that he always plays on Hard Mode and never uses the "Run" command, and he is stated to have nerves of steel in order to accomplish this. He came up with an original theory that enjoying the extreme thrill of cornering yourself with all exits sealed is the definition of fun. However, he also enjoys more casual things like getting chased around as if it's a game, at least by people like Kaede, Kirumi, and Shuichi who wouldn't beat him up after catching him, and loves hide and seek. Later in the game, it's shown that he has drawn multiple childish drawings with silly fantasy concepts such as ray guns with rainbow-colored beams (along with a silly self-portrait of himself), and he apparently owns a toy helicopter and a supercar. His favorite presents include a pop electro-water gun, most likely to use for pranks, and a hammock to relax and feel like a child again. In his Free Time Events, Kokichi makes multiple references to shows such as Yu-Gi-Oh! and Tobaku Mokushiroku Kaiji, and it's indicated that he likes the Shōnen genre. His supreme leader claims can also be rather childish at times, like destroying cities with giant robot dinosaurs. His Research Lab had a stereotypical dramatic throne of an evil ruler, comical instead of harmful objects, flashy cars and was even described to be like a child's idea of what an evil supreme leader is. In his Love Suite fantasy sequence, it's shown that he likes to imagine himself as a legendary phantom thief who has a romantic rivalry with a detective, and it's indicated that while he loves the chase (even more so than committing the actual crime) he is also very excited about being caught and punished. In the official art book, it's stated that he is quite nonchalant about his hair, but he does possess a true sense of fashion he keeps hidden and displays through his colorful underwear, supposedly not finding it fitting with his supreme leader image. Notably, he also keeps referring to Kirumi as his "mom", and he is a big admirer of Santa Claus. He himself has openly admitted that as a teenager he is, of course, immature, and he claims that people should never lose their childish side. However, at other times, he acted offended at the mere suggestion that a supreme leader such as himself would do something so childish (though this reaction, too, often appeared to be just a joke).
Above all, Kokichi considers lying a big part of himself and a way of life, appearing either unwilling or unable to stop it and even claiming that his body is made up of 70% lies. He frequently claims something only to shortly afterward reveal that he was just lying, making it very hard to trust his word. Sometimes the lies are very obvious, but at other times it can be near impossible to tell whether or not he is telling the truth. He also tends to say whatever riles up the other students at the moment and tries to mess up conversations without any apparent reason. Indeed, neither because of impulsivity or the need to confuse others, he lies mostly because he feels he's free to do whatever he wants whenever he wants. He has also frequently pointed out that even if he told the truth, the others wouldn't believe a liar like him, indicating that he uses his constant lying to mask his own true feelings and intentions. Furthermore, being a liar himself, he always seems to notice when and why other people are lying. Ironically, he appears annoyed if other people are lying, either to others or themselves, and he claims that he hates liars. He also dislikes it when people claim they hate lying yet they lie . He sometimes does not point out other people's lies and may even go along with them simply for the fun of it, but he seems to enjoy revealing other students' more serious dishonest ulterior motives with near malicious glee, such as when he exposed Maki as the Ultimate Assassin and called Kirumi out on her attempt to guilt another student into taking her place for the execution. Notably, whenever his face and tone goes more blank and neutral, it's heavily implied that he has a rare moment of being completely genuine and honest, and he has an occasional habit to unintentionally speak out loud what he is genuinely thinking and sometimes even does it intentionally by openly explaining it to the other students.
Besides lying, Kokichi is shown to be very intelligent in general, with he himself claiming that his head is always filled with complicated thoughts. He is capable of creating complex plans in a short amount of time—a notable case being the time he wrote a script for Kaito during the class trial, which was written thoroughly about possible reactions and possible routes of how the trial will play out. He is also very perceptive, possibly due to being a major liar in a game that's main theme is lies, as he often appears to have a deeper understanding of the situations and can predict things to the point of resembling breaking of the fourth wall. He quickly figured out Kaede's major flaw, foreshadowed her death and explained to her that the definition of common sense varies depending on the person. Notably, he understands that in order to truly win the killing game you must end it, not necessarily survive it. During his free time events, he even gives this advice to Shuichi, telling him that games can be won by not playing them. He also does lots of investigation alone without other people noticing, analyzes the other students, writes their fates and his impression of them on his whiteboard, and hoards lots of evidence from the past cases into his room. Supposedly because of his intelligence, Kokichi strongly disliked boredom (though not as badly as Junko Enoshima and Izuru Kamukura). He tended to complain about situations he found boring and even during some bad situations he noted that it's fine because at least it's not boring. If he liked a person or found them interesting, he tended to state that they are not boring, which was a big and rare compliment coming from him. Furthermore, his last words even include a prominent line "At least I... wasn't boring, right?", showing that the matter appears very important to him. In general, he also seems to have an interest in "useful" people and wishes to have such people around him. It's implied that contrasting his intelligence, his weakness might be his physical strength. He dislikes heavy physical work and sports and complains whenever he has to participate in such activities, and he himself admits multiple times that he is weak physically. He also appears fearful whenever others threaten to hurt him, often running away quickly or relying on Gonta for protection. During Chapter 2, he hides in the casino from Kaito and gets startled by Shuichi's appearance as he mistakes him for Kaito for a second. However, it should also be noted that he was seen as incredibly agile by Maki when he punched Kaito at the end of Chapter 4, and he further claims that he lied about not being good at fighting, so it is unclear just how capable he really is when it comes to a fight. He claims that as an evil supreme leader, he doesn't like violence and would rather sneak around.