A shy, awkward, and withdrawn 16-year-old girl. Natsu has difficulty expressing herself and was always put down at home and bullied in school. She becomes essential to the team's survival by observing and writing down what they discover, such as which foods are edible. She has conflicting feelings for Arashi, knowing that he has a girlfriend. She is picked on or rather, teased by Semimaru frequently. Natsu means summer and iwashimizu means water trickling from rocks.