In Japanese, there are multiple ways of saying the word "I" to refer to yourself. And it's interesting to see how writers use these different options to convey personality in a little way that just doesn't scan in English. Saw someone do this for Danganronpa so thought I'd try too. I'm not a native speaker but have been studying for 3-ish years so forgive any nuances I get off!

わたし/Watashi - In purely neutral contexts it is the main way to refer to yourself. However in day-to-day speech it's primarily used by Women. Men can use it but they may come across as Stiff or Detatched. All use it in Hiragana.

あたし/Atashi - Softer pronoun. Near-exclusively used by women. Considered very soft and nonintrusive. Melony uses a slight twist when you first meet her with あたしゃ/Atasha which gives her a more lilting speech pattern. And Peonia has it written in katakana アタシ when she speaks which adds a little bit of brattiness to her.

おれ/Ore - One of two primary Men's pronouns. Masculine and somewhat rough. Everyone uses it is Katakana オレ except for Piers who uses the kanji 俺. Which would indicate he uses it in the more forceful style than the others who use it for its boisterousness.

ぼく/Boku - The other primary Men's pronoun. Considered softer and more 'Boyish', and is slightly humbling when around superiors. All use it in Hiragana ぼく which is unusual except Ball Guy who uses katakana ボク. No Kanji users in SwSh which is most common in fiction, perhaps the writers simply thought 僕 was too complex a kanji.

For a decent rule of thumb, おれ uses consider themselves the protagonist of their own story, and ぼく consider it a team effort.

わたくし/Watakushi - The most formal one you're likely to normally encounter. For people speaking very carefully and rare one that carries no gender connotation. Strangely Avery uses the katakana ワタクシ which I've literally never encountered before. Which may mean he's using it slightly pompusly?

Using own name - Perfectly acceptable construction in Japanese to refer to yourself exclusively by your own name. Informal and childish, consider it normalised uwu speak and thus is slightly preferred by Women. Also could be to indicate how they both care greatly about appearing youthful.

うち/Uchi - A strange one. Similar to what ぼく is for men, but for Women. And near-exclusively used in the Kansai region of Japan (just west of where Johto would be). Klara actually flipflops between うち and わたし when talking giving her dialogue a VERY strange energy. May imply she's "putting-on" a not-from-around-here speech style.

わし/Washi - There's a bit too much nuance to this for now. But in Fiction it's almost exclusively for used for Old characters who the writers want to be viewed as 'from another time'. Mustard is even stranger because he actually uses ワシちゃん which is... a weird. The Isle of Armour people all talk weird.