Complex sentence: Many people underestimate her athletic ability because of her work ethic in school, which makes her seem like a person whose only interest is studying, but in the cases of hostile situations, she will fully display her expertise in martial arts.
Split into simpler sentences: Many people underestimate her athletic ability because of her work ethic in school, which makes her seem like a person whose only interest is studying. However, in hostile situations, she will fully display her expertise in martial arts.