Nagisa is 18 years old belonging to a new race called the Dumans. She may be polite but she lacks common sense, which can make things go sour unintentionally. It’s not that she does this on purpose, she doesn’t really mean any harm. The reason she is this way is because she’s been fighting all of her life.
Her purpose is to complete a very big mission, this will be revealed within Story Mode.
Source from Wikipeida