Natsume is good humoured, but rather over the top in terms of personality; she can be seen as a more extreme version of her older sister. She does not hold grudges, but considers herself a friendly rival of Anita King as classmates and one who shares a close relationship with a writer.
She greatly admires her older sister, Haruhi, and is frequently promoting her sister's novels at any given opportunity.
After the British Library's experiment on Nishihama, Natsume's personality changes. She is more subdued and embarassed by her sister, offering more modest opinions about her sister's work. This is in line with the Special Operations Division's ideal world: Natsume's personality before was somewhat irritating to other students, but after the change she gets along well with them.