They all have different meanings, representing various speakers' characters.
① 私（わたし）
① 私（わたし）
Name: セバスチャン・ミカエリス（Sebastian Michaelis）
Gender: Male
Comic: 黒執事（Black Butler）
Age: ??? （謎）
Character: Courteous as a Butler, Devilish
Name: 神楽（Kagura）
Gender: Female
Comic: 銀魂（Gintama）
Comic: 銀魂（Gintama）
Age: Still in adolescent stage
Character: Strong, caring, have girlish thinking but can fight violently because of her "Yato" blood (Strongest and the most bloodthirsty alien race)
② 僕（ぼく）
Name: 常陸院 光（Hitachiin Hikaru） 常陸院 馨（Hitachiin Kaoru）
Gender: Male
Comic: 桜蘭高校ホスト部（Ouran High School Host Club）
Comic: 桜蘭高校ホスト部（Ouran High School Host Club）
Age: １５（双子）
Character: mischievous, devious, childish young twins
Name: 志村新八（Shimura Shinpachi）
Comic: 銀魂（Gintama）
Gender: Male
Age: Teenager
Age: Teenager
Character: Generally coward, like an ordinary boy on the street, grumbler, otaku（お宅）
③ 俺（おれ）
Name: 鳳 鏡夜（Ootori Kyouya）
Comic: 桜蘭高校ホスト部（Ouran High School Host Club）
Gender: Male
Age: １７
Age: １７
Character: realistic, pragmatic, sly, tyrant, cares about the profit of the Host Club very much, mostly cool, sometimes caring
Name: 坂田銀時（Sakata Gintoki）
Comic: 銀魂（Gintama）
Gender: Male
Age: Supposed to be in his thirties
Age: Supposed to be in his thirties
Character: just, very strong, has savage swordmanship, protective towards friends, lazy, can't live without sweets, loves to read comic magazines
④ あたし
Name: 寿 蘭（Kotobuki Ran）
Comic: GALS!
Gender: Female
Age: 高校生
Age: 高校生
Character: "Spicy girl" in 渋谷 (Shibuya), Hates study, sports maniac, value much on justice and friendships
Name: さくら ももこ（Sakura Momoko）
Comic: ちびまる子ちゃん（Chibi Maruko-chan）
Gender: Female
Age: ８
Age: ８
Character: ordinary primary school girl, always avoids homework & housework and takes advantages of her doting grandfather
Atashi is the adapted version of watashi, of which the "w" sound is omitted. Atashi is generally used by female, especially girls. It makes the girls sound cuter. "Atashya" was originally "あたし は", normally young children will make it brief.
おりゃあ、ぼかぁ、わたしゃ、わしゃあ、おらぁ are also from the same case.
⑤わっち
Name: 月詠（Tsukuyo）
Comic: 銀魂（Gintama）
Gender: Female
Gender: Female
Age: Unknown, probably in her twenties
Character: Independent, already gave up her identity of woman, protective to people whom she loves, head of Ninja Protect team of 吉原桃源郷 (Yoshihara Underground Street)
Wacchi is also an adapted version of watashi. However, it is not common among the usage of women language for it is supposed to be the language of women (prostitutes) in Edo Period. Let's see a comment on Tsukuyo from other female characters in Gintama.
"According to Toujyou's investigation, She [Tsukuyo] uses 郭詞（くるわことば）"Wacchi" as personal pronoun."
"Wacchi!? Wait! What kind of girl is she [Tsukuyo]? Miss Otae, what kind of girl is she[Tsukuyo]? This girl[Tsukuyo]!!"
"Huh? She really strikes hard to act her character. She doesn't have to say this language grudgingly~~"
They don't like the personal pronoun "wacchi" as it means that the speaker is a prostitute. However, Tsukuyo is not a prostitute, she is just living in that street full of prostitutes and protects the women there from the violent act of the male customers. Therefore, use of personal pronoun can also represent the origin of the speakers.
⑥あっし
Name: 外道丸（Gaidoumaru）
Comic: 銀魂（Gintama）
Gender: Female
Age: Unknown
Character: 式神(shikigami), protective to her master, merciless to other creatures
**"Asshi" is one of the adapted version of "Atashi", which is normally used by 職人 (people who have their specialities used in their work).
Name: 華陀（Kada）
Comic: 銀魂（Gintama）
Gender: Female
Age: Unknown
Character: Glamorous appearance, devilish soul
⑧我／吾（われ）
Name: 椿 平子（Chin pirako）
Comic: 銀魂（Gintama）
Gender: Female
Age: Teenage girl
Character: 人斬り(killer), obeys her master.
⑨余／予（ヨ）
Name: 紫 劉輝（Shi Ryuuki）
Comic: 彩雲国物語（Saiunkoku Monogatari）
Gender: Male
Age: Around 20
Character: determined to be a good king, loves Shuurei very much
** "Yo" is the singular pronoun used by the 藩主(Hanshuu) and 君主(Kunshuu).
わたくし(Watakushi) should be the most polite form of personal pronoun, followed by わたし(Watashi).
They can be used by both male and female. For most of the time, if not in a serious situation, male will use ぼく(Boku) or おれ(Ore) ; female will use わたし(Watashi) or あたし(Atashi). Young boy and young men would like to use ぼく(Boku) while mature and strong men would use おれ(Ore).
わっち(Wacchi) , あっし(Asshi) , われ(Ware) , わし(Washi) , 余(Yo) would rarely be used in modern Japan because these are rather old languages. The elderly or people in rural area may still use it.
I'll prefer あたし because it is more casual and sounds cuter than わたし. However, if it's a very serious occasions like interviews, I'll prefer わたくし.
There are still a lot of different Japanese personal pronouns which are not found.
For further details, please refer to:
日本語の一人称代名詞 - Wikipedia
