Her voice should be medium high or even just high in general but soft. Make her sound caring but firm as Yoko is seemingly a damsel at times but is a strong lead.

(Copied from Wikipedia)

Yoko Littner(ヨーコ・リットナーYōko Rittonā), the series' lead female character, is a young woman from the village of Littner. She has considerable combat experience even before her chance meeting with Kamina and Simon. In Episode 1 a Gunmen crashes through the roof of Giha village and prepares to attack the populace when Yoko, who has been tracking the Gunmen, rappels down a cliff whilst firing her electromagnetic rifle. Her solo attack disables the Gunmen, giving Kamina and the villagers time to seek safety. Kamina, however, chooses instead to make advances at Yoko and initially diminishes her combat qualifications on the grounds of being a woman, yet acknowledges "that lady's spunk".