Japanese by Example
Learning through examples in manga

アタシ from an Osakan girl

Manga panel from 名探偵コナン showing example of アタシ from an Osakan girl.
名探偵コナン » Volume 19 » Page 88

Heiji invites Conan, Ran, and Mouri on a tour of Osaka. While Heiji steps away, his childhood friend who’s been following the group confronts them. She opens by saying “You’re Kudo, right?” and that she heard it from Heiji. Conan, realizing his true identity is about to be revealed to Ran, tries to stop her.

コナン:
「あ、あ、ちょっ…」
“Ah, ah, wai--!”
和葉(かずは):
「とぼけたかてアカン! アタシにはもう全部(ぜんぶ)わかってんねんから…」
“It's no use playing dumb. Since I already know everything...”

The choice to use katakana for Kazuha’s アタシ could be to further emphasize her Osakan accent.