Nagisa is a calm, collected, and friendly individual who can be exceptionally observant and circumspect of his surroundings and the situations that develop around him. He tends to pay strict attention to Koro Sensei in hopes to understand the motives of this mysterious being and compiles the information he has gathered in his notes for reference.
1) Nagisa is one of the few people in the entire manga who is referred to by his first name (similar to Karma) when nearly everyone else is referred by their family name. According to Koro Sensei in his talk with Nagisa's father, it is because Nagisa asked everyone to call him by his given name so he could easily make the transition back to his old name if his parents ever ended up getting back together.
2) Nagisa was called "Gender" due to his feminine appearance during the class 3-E training using code names. Nakamura was the one who gave him his codename.
3) Nagisa's favorite subject is English while his least favorite subject is Science.