Mayoi Hachikuji is a girl Araragi meets in a mysterious episode concerning the 'Lost Snail.' She looks to be about 12 years old, but is that really the case?
She can be quite brash or harsh but eventually Araragi rubs off on her and she starts to pick up some of his more 'lewd' personality traits. She does have an older form in an alternate timeline episode of the Monogatari series, so bear in mind you may have to do an older voiced version of her.

These line directions and example lines may or may not be representative of what appears in the final product.

Line Directions:

Line 1: Loud, but not quite shouting, more a raised voice, slightly panicked scenario.

Line 2: She's yelling at the main character, but also teasing him about his name

Line 3: A famous Darth Vader quote, but if you haven't seen Star Wars IV - A New Hope, either give it a quick YouTube, or try and say the line somewhat slowly with a thoughtful emotion

Thanks for looking and good luck!