Was often bullied in school, which is why she eventually quit going to school and became a hikikomori.
Her family appeared to be kind and considerate of her; her mother appeared to be especially encouraging, however she seemed not to care much for them and was more concerned about her pet cat, Nuts. Natsu was the very first character to be introduced in the starting chapter of the series, "Slow Awakening". She woke up on a ship and fell into the water because of her disorientation, but was saved by Arashi.
When the group arrived on Cat Island, Natsu's shy and awkward-clumsy personality caused her to be scorned by Botan and teased by Semimaru. She kept close to Arashi who was friendly to her and ran away together with him when he started distrusting Botan.
Later on, they involuntarily reunited with Botan and learned about the truth of the New World. Natsu then realized that Cat Island wasn't at ocean but in the middle of a sweet water sea. This enabled the group to navigate to the main land of Japan.
When Arashi was stung by a black fever wasp in the Kansai region, she ran around searching for help and came across Team Autumn. Akio and Ran separated her from the boys to question her. Suspecting that she might be sexually abused by Akio, Arashi and Semimaru followed her but got caught.
After their successful escape from Team Autumn, Natsu crossed the ocean and arrived at Southern Honshu. When Semimaru silently grieved at his mothers' grave, Natsu tried to quietly console him by searching for floral offerings. During Arashi's fall into despair after realizing Hana was probably dead, Natsu gave it her best to cheer him up again going as far as to even perform a Manzai act together with Semimaru.
They found a ship hidden in a shelter and returned to Bungo Fuji. On their way, Natsu discovered Hibari floating on the ocean. The young clairvoyant warned them that Hotaru - and probably the others as well - were on the verge of death.
Travelling back to Team Autumn's village they found it in ruins, but Natsu discovered a letter left behind by Hana. Overcome by jealousy, she decided to hide it from Arashi but was eventually discovered by Semimaru who made Natsu realize that maybe she wasn't really in love with Arashi, but only head over heels for him because he was the first person who had treated her kindly. Natsu then gave back the letter to Arashi and felt cheered up when she saw how happy he was.
Afterwards, she continued her travels with Team Summer B and surprisingly became sort of the new pet of Ango, who saw her as the replacement of Shigeru. Ango encouraged her to practice the Whip-off-the-tablecloth trick.
When they set off to continue their travels, Natsu realized that Semimaru was in trouble and rescued him just in the nick of time. She cried a lot, being really glad that he had survived. Arashi took it as a sign that she might have started falling for Semimaru.
At the Fuji, Natsu made her greatest internal development so far when she for the very first time wasn't the one to be rescued, but decided on her own to discover a way to rescue Ango and Arashi who had been caught in a tight spot. This made her a bit more self-conscious and courageous.