Seiko had a reserved and pessimistic personality and a stilted way of speaking. She also appears to show little compassion or optimism over changing the remnants, believing them to be rotten. Much like Kyoko Kirigiri, she was conscious of how easy it was for people to guess what she was thinking through her facial expressions, so she hid her face behind a mask. She got startled easily and appeared quite anxious, and, during her time in high school, she was shown to be quite shy as well. Despite having very little compassion in general, Seiko was shown to be one of the most understanding and compassionate members of the Future Foundation, as she showed remorse over both Chisa Yukizome and Daisaku Bandai's deaths and was angry with herself for not being able to save them. During high school, Seiko was described as soft-hearted, and she often took requests from other students such as Teruteru Hanamura, Nagito Komaeda, and Ruruka Ando. She also loved animals, providing care for an injured animal at no charge, as well as producing several medicines specially formulated for use on animals. Because of her soft nature, Seiko (similar to Mikan Tsumiki) was very loyal and forgiving to friendly or helpful people towards her. Because she could not eat Ruruka's sweets due to her medicine, she felt guilty and tried to compensate by helping her whenever she needed it.

As a pharmacist, she had an extensive knowledge of medicine and drugs. She created many different kinds of drugs, including ones that can drastically change one's physical appearance and make them much faster and stronger. She was always very meticulous while dealing with these medicines and drugs, and tried to work in quiet environments so she didn't get disturbed or lose focus when working.

Voices for Reference (ENG): https://www.youtube.com/watch?v=AljfHCN0Y_Y