Nagisa Shigetsu is a character in Danganronpa: Ultra Despair Girls, and his title is the Lil’ Ultimate’ Social Studies. A clam, level-headed, and intelligent young boy, Nagisa is the most serious of the Warriors of Hope. He often plays the voice of reason when the others get into arguments. He doesn’t like to be underestimated or looked down upon because of his age, so he tries to avoid childish subjects or items to ensure that. Nagisa is extremely humble - he’s super loyal and protective of his friends and the other children. He’d often downplay the abusive home life he had to experience in comparison to the other Warrioes, often putting their struggles before his own. Despite what he and the others have done though, he still feels a lot of guilt for it - he knows morally what they’re doing is wrong (showing his level-headed nature), but feels that they had to be done - including killing his parents. Despite the abuse he endured from them, he still feels in debt to them for raising them - though he hates it, he can’t stop feeling respect for them. 

Nagisa is an extreme perfectionist. His whole life, he wanted to exceed the expectations of everyone to prove his worth not just to himself, but to his family - however, this caused placing expectations on him to be a bit of a trigger for him. That trigger would propel him to try to exceed those expectations he’s given, to the point he’d sacrifice his health and well-being. Nagisa suffers from a fear of abandonment because of his traumatic childhood, so even if he feels weak or sick from the work he does, he still try to achieve his goals if it means he gets the validation he’s looking for.

Throughout the game, it is revealed that he has a crush on Monaca, often losing his composure when she does things like complimenting or hugging him - but knowing Monaca’s true nature, she uses this to manipulate him to do what she wants. The others constantly tease him about it and accuse him of acting “like an adult” for these feelings, to which he tries to become embarrassed and defensive about it. However, whether he wants to admit it or not, he still has that curiosity about it.

The creators of UDG have stated that, despite his maturity and intelligence, Nagisa is also the most “childlike” personality-wise, most likely referring to how Monaca could easily manipulate him and how his admiration and affection for her blinds him from her negative traits, and tainting him to think she’s a sweet girl until she reveals her true self. Even then, he still couldn’t be worried about her in Dr3: Future Arc.

Voices for Reference (ENG): https://www.youtube.com/watch?v=rBhKbmRkdLY