Shinako is the eldest Sugimoto sister.

She is straight-forward and very blunt, though extremely perceptive and easy-going. She seems to dislike conflict finding it dull and unnecessary, and is rarely ever surprised by people's actions. She appears to enjoy being amused at other people's expense.

She is an alumna of Fujigaya Girls' Academy, and during her high school years, she was very well known. Through flashbacks from various characters, it is revealed that Shinako's nature had not changed very much from when she was a teenager.

She does not care about people's sexual orientation and openly asks Fumi if she's a lesbian or bisexual when she first meets her. She has said herself that she had many "girlfriends" growing up, though this may be only due to her being bored rather than personal attraction to other women. She is an unrelenting flirt though she is also very aloof, a combination which has caused her to seem slightly cruel to those she was ever involved with romantically.