Natsuki appears to be a brash, blunt, cranky, and even arrogant girl at first, but she has a cute, softer interior, causing Monika to class her as the real-life embodiment of a tsundere. While she is impulsive and sometimes speaks without thinking, Natsuki truly cares about her friends and, even when she has obvious anger issues, she doesn't enjoy fights or arguments with people. Over the course of the game, it is shown that she worries about Yuri, and, on one occasion, she gives the protagonist a note asking him to help Yuri, fearing that if she spoke out then it would cause more arguments. Natsuki is very stubborn and has a hard time expressing how she feels and what she wants. When repeatedly challenged, she often becomes awkward and tongue-tied, then aggressive, and then simply bursts into tears. (Description pulled from the fandom wiki)

Natsuki is your classic tsundere.  High in pitch, but not hesitating to spit out rude or callous remarks.  She has walls that she is usually unwilling to let down, and that should be present in your performance.