"Terezi can be civil, flirty, manipulative, and derisive in her interactions. Her drive for justice results in some callous and cruel actions, the majority of which she has been shown to be deeply affected by and regret later on. She is almost always grinning and in many panels it appears as though she is cackling madly. From her conversations with the kids, Terezi's attitude differs depending on who she is talking to: she acts helpful towards Rose, dubs them 'hatefriends' and tells her about Sburb. This approach may be a part of her trolling of John by pretending to be helpful towards him (through insulting him and using a kind of angry politeness) and giving him advice that leads to his death in the alternate timeline. "

(Taken from the Homestuck Wiki)

For Terezi, she would have a raspy, but mature voice. Think of it being almost like Peridot’s voice from Steven universe in a way, but a lot less nasally.