About me: Nagito first comes off as a polite, friendly, and easygoing, yet somewhat insecure boy. Hajime Hinata even mentions that his bright smile is very comforting. He was the only one that stayed with Hajime while he was unconscious, and expressed concern for his well-being, even after Hajime told him to go away. Though friendly with everyone, he is mostly interested in spending time with Hajime and can be quite clingy, and as a result, Hajime considers him a little annoying and not the most reliable, but still thinks he's a kind person.
Nagito attempts to be the team's conflict mediator and a poster boy for optimism, often encouraging others to be hopeful and to cooperate, however, he isn't taken very seriously. Hiyoko Saionji makes fun of him and considers his dramatic pep talks about hope and friendship embarrassing and tiresome. He is generally quite submissive and has a self-deprecating attitude, even agreeing with Hiyoko and the others' snide remarks.
Although he claims to be a pessimist, Nagito often appears unusually carefree; he lacks tact and occasionally says awkward, strange, and unsettling things without realizing it. At times he can also be more forceful; he is quick to scold Hajime when he's being too pessimistic or when he thought he was bullying Mikan Tsumiki. He is also notably annoyed by Teruteru Hanamura's treatment of girls and takes it upon himself to keep an eye on him to make sure he doesn't harass anyone.
During the first Class Trial, Nagito is revealed to have a twisted obsession with hope. As a result, he often begins to ramble on about hope, sometimes rather abruptly—just talking or hearing someone else talk about hope or speak in a hopeful manner can render him in a state of bliss, causing him to sweat, and hold himself in order to contain his excitement. Furthermore, he sometimes has sudden outbursts in which his eyes darken and start to swirl (Hajime describes the look "a crude mix of hope and despair") while he rambles even more enthusiastically, but he usually snaps out of it fairly quickly and seemingly isn't aware of what happened.
He believes hope is an absolute good, and any deed done in the name of hope is righteous, even murder. He feels no guilt for hurting others in the name of hope, as he believes that hope will always win in the end and justify the means. He believes there are constant clashes between two or more hopes, and that "strong hope" will devour "weak hope" (adoringly, he calls this process "poetry in motion"). To him, ordinary untalented people, despair, and weak hope are just stepladders for creating stronger hope, and he considers these things necessary, while simultaneously having aversion to them. Nagito believes that the Ultimates are chosen at birth, and are capable of strengthening their own hopes by facing and defeating despair, instead of being broken by it like weaker, ordinary people. Like Hope's Peak Academy, Nagito strongly associates hope with talent and talent with worth.
Nagito desires to help to create the absolute hope that can overcome any despair, and he believes that the Ultimates can become embodiments of hope. However, he doesn't consider his luck to be a true talent and thus doesn't consider himself an Ultimate (despite this, he has also mentioned that his luck is the only good thing about him, and he sometimes finds it very useful). Instead, he desires to be a "stepladder", and sacrifice himself so the other Ultimates can shine. Considering himself a lower being, he often feels shame just for being in their presence. He confuses his obsession with the Ultimates' hope with pure, platonic love for his classmates and he genuinely believes he's doing good for everyone's sake. He has a strong desire to be useful and he is very self-sacrificing, but his ways of helping are often very wrong, harmful and potentially very dangerous, as he is willing to cause temporary despair in order to have hope born from it. He has little regard for his own well-being and doesn't care for his own life, and encourages the other students to kill him to achieve their own hope. He believes that dying for the sake of hope is an honor and the only way for "someone like [him]" to be useful to the world. In his Island Mode ending, Nagito reveals that he wishes to die so he can finally be free from the suffering caused by his luck. However, he is strongly against dying a meaningless death or dying for the sake of despair.
After discovering that he and his classmates were the Remnants of Despair, Nagito began to act arrogant, impatient, and disrespectful towards his former idols. He had particularly little patience for the slow-witted ones, including Kazuichi Soda and Akane Owari. Nagito realized that by killing his classmates and therefore destroying the Ultimate Despair, he could himself become the Ultimate Hope. Though he claimed it was for the sake of the world's hope, it's also implied he wanted to become Ultimate Hope so he could finally be appreciated, even if only after his death.
Nagito believes his views on hope are logical and extremely obvious and has trouble relating to others when they disagree. Though he is very stubborn, he has changed his ways of thinking during the series. He is sometimes conflicted between his extreme beliefs and his own feelings. He calls himself "worthless trash" that doesn't deserve any kindness from others, but at the same time, he desires to be appreciated, loved and understood. In his fifth free-time event, Nagito revealed that his true, biggest wish is to be loved by someone at least once in his life, and to not die alone. Furthermore, even though he greatly admires and claims to love all of the Ultimates for their talents and capability to embody hope, he doesn't really care about them as individuals. He isn't truly saddened by their deaths, and even appears to dislike some of them as people. Similarly, he may not have anything personal against the talentless—in fact, the only student Nagito genuinely cared for is Hajime, a talentless reserve course student.
In Super Danganronpa 2.5, based on what the World Destroyer said about his illusionary world reflecting his desires, Nagito may actually have a hidden hatred towards talent and wishes for a world without talent because he believes the world and its people would be better if talents were taken away so everyone can become equal and free. For example, Kazuichi's illusion form reflects his insecurities about not being good enough for some people's company. In Danganronpa 2, Nagito is seen showing interest in Hajime because he doesn't feel like an Ultimate to him and thus they're more similar and equal in his eyes. His wish to live an ordinary life with someone is also mentioned in Zansakura -zanka-, which is a song about his inner feelings. In the OVA, Nagito was both shocked and embarrassed to realize that he may actually have such emotions and thoughts, as he usually claims that only those with great talent can bring great hope. Naturally, he was relieved that nobody else saw this side of him during his comatose state. Despite learning about this hidden side of himself, he refuses to accept it and instead continue with his desire for hope. In the end, he is shown taking Hajime's hand and deciding to move towards hope, implying that being with Hajime is his hope.
Nagito has a laid-back and quite calm demeanor, and he is often smiling or laughing. However, his cheerfulness may not be entirely genuine, as his calmness is more akin to apathy and he tends to feel strong self-hatred. He remains cheerful even during his self-loathing rambling, and acts disturbingly optimistically toward horrible things such as death, as long as it's for the sake of hope.
Nagito likely suppresses any real feelings of sorrow or fear as a coping mechanism, because he experiences tragedy often due to his luck. He has also seen very unlikely and strange things due to his luck, which is why he is not easily surprised. As a result of everything he's been through, he has become more emotionally numb and he is rarely openly angry or scared. He often lacks appropriate emotions in situations where most people would be afraid, like threats of violence. Even so, he once mentioned that he feels fear every day of his life, most likely because of his luck. Furthermore, he can be very angered by despair and other things he considers insulting toward hope. When he does express anger, he becomes notably more serious. When displeased, he is very passive-aggressive.
Nagito is capable of feeling some guilt and care for others, and he does not truly enjoy the suffering of others, but the hope born from it. Still, he is typically very emotionally cold and doesn't seem to feel much remorse or grief neither. He's shown to be fine with tricking, manipulation, and stealing, though it should be noted that he does these for the sake of a bigger goal rather than his own ends. He has been described as a very honest person, though he will lie if he finds it necessary. While he prefers to do things on his own, he is very lonely and often tries to join the other Ultimates in their activities and becomes really happy if he's given attention, even if just a little bit. It's also implied that he intentionally keeps his distance from others because he wants to protect them from the disasters caused by his bad luck.
Nagito is also one of the most intelligent characters of the second game, being very cunning and smart. He often solves a case long before the others, but he doesn't want credit for this. Instead, he often acts ignorant during the trials, manipulates the conversation by giving (often unnoticed) hints and then praises the others as if they're the ones who figured it all out. He does this because he considers himself too unimportant to take a more openly active role, wants to see the other Ultimates figure things out by themselves, and he wishes to remain neutral until he can determine which side has the stronger hope.
Overall, Nagito appears to be somewhat out of touch with his own feelings. He tries to be polite, but he can be overly harsh and insensitive, often without realizing it. He has a lack of social awareness and insight into his own behavior. Because of this, he has problems understanding how his own behavior can affect other people and he may not understand or pick up on social hints. He struggles with expressing himself in a socially appropriate manner and this often leads to people misunderstanding what he's trying to say, and vice versa. During Chapter 1's additional dialogue, Nagito explains that he says dubious things without meaning to and that he realizes this, but can't stop it from happening. However, he also adds that he always sincerely means what he says.
In Chapter 4 of Danganronpa 2, he accidentally offended both Kazuichi and Fuyuhiko Kuzuryu—in Fuyuhiko's case, he understood that he said something wrong and quickly apologized, but remained quite confused, and misunderstood what he said wrong. For Kazuichi, he was completely oblivious and only noticed that Kazuichi seemed angry for some reason. Other examples of this type of behavior include his odd and awkward sense of humor, often making jokes at inappropriate times, like soon after someone's death. He usually backs off if the others explain to him that his behavior is rude or he tells them he wasn't being serious. Similarly, when he tries to compliment someone, it can come across as an insult. His gauge of sarcasm is quite poor, too, as he sometimes mistakes mean comments for compliments. He also doesn't understand why the others are paranoid, angry and afraid of him. He's often confused by the others' behavior, thinking that they're strange for acting so suspicious of him, to the point that he sometimes worries about their mental health. He thinks that the others hate him because he's a lower human being.
In his final free-time event, Nagito claims to have Frontotemporal Dementia, a neurodegenerative disease that affects their personality and social cognition and thus may account for some of his strange behavior. He claims he was lying immediately afterward and it is unknown whether this was a lie or not. Nevertheless, it is clear that he has some type of psychological disorder.
It should be noted that Nagito's strange luck affects his condition as well, seemingly allowing him to live much longer than he should have, and likely giving him unusual symptoms.