Tomoe's personality seems wild, active and ambitious. She enjoys traveling and has traveled alone across the continent of Eurasia at the time of her college enrollment, and even had the experience of climbing a 2000m mountain. As a martial arts expert in Aikido and Taiho-jutsu and a proficient university student, she "was not content with conquering Japan alone, and had decided to go out and challenge the world."
From the letters she wrote and the interactions she has with her brother, Hōtarō, at home that she is also a bit of a hasty and pushy individual. Whether or not this pushiness is something she also does to others is unknown.
Based on her interactions with Hōtarō again, she can be a rather flaky and/or irresponsible individual at times, as shown when Hōtarō asks for a good luck charm, she gives him a broken fountain pen.
Tomoe has long brown hair and seems to be a bit shorter than her brother Hōtarō. Her eye color is somewhat greenish or could be the same color as Hōtarō. Her face is almost fully revealed and it was shown during the Kanya Festival when she's about to check how Houtarou's doing. She wears a variety of styles for casual clothes from simple tops and short pants at home to colorful attire when going out.
While much is not known about Tomoe yet, she plays an important role in the series, often contributing to the circumstances her brother Hōtarō would face at school. By the start of Hōtarō's second year in high school, Tomoe is in Banaras in India, en route to Sarajevo in Bosnia and Herzegovina. In one of the letters she habitually sends to Hōtarō, she told him to join the Classics Club. Although Hōtarō is quite lazy and refuses to join, he eventually reconsiders because he knows that his sister "would make it very painful to refuse". Eventually, her letter would allow the Classics Club to form once more, and she intervenes once again, this time in a case involving the issue of the Hyouka anthologies of 45 years ago, when she mentions the location of old anthologies from the Classics Club.
Tomoe maintains contact with Hōtarō by mail or by phone and later pushes Hōtarō in a case involving the amateur film of Class 2-F for the Kamiyama High School Festival by referring him to the third year Fuyumi Irisu. She later warns Irisu about messing with Hōtarō following the developments of the case.
Soon enough, she comes home, just in time for the day of the school festival. Tomoe even participates in the "Straw Millionaire" situation Hōtarō found himself in, by giving Hōtarō a fountain pen, and later trading Hōtarō's hand mirror for a copy of "A Corpse at Night", which Hōtarō later required to solve the Juumonji Incident.
Throughout the novel, she travels around the world, from New Delhi to Beirut and so on. Readers also learn that Tomoe asked her little brother to join the Classics Club because she was a member in the past. It seems that she has a connection with Jun Sekitani because, in a brief phone conversation, she is able to give Hōtarō a clue about the uprising Jun Sekitani was connected to 45 years ago.
As his older sister, Tomoe has a warm family love for him, even though Hōtarō himself is not fond of it and normally acts more bored, in a possible attempt to ignore her. However, he seems to fear her in other ways, such as knowing that he'd face "something painful" if he tried rejecting Tomoe's request of joining the Classics club.
Due to being able to give some hints to Jun Sekitani's connection to the events of 45 years ago, Tomoe may have possibly been an acquaintance.
- In the Kanya Festival, Chitanda and Tomoe share eye contact in the hallway, with Chitanda following with: "I feel like if I've seen that look before." This may be a hint that Tomoe's eyes resemble Hōtarō's.
- In End Credits of The Fool, Hōtarō mentioned that the book about Tarot Cards from his bookcase belongs to his sister. Since Tomoe was part of the Classics Club, most of the books probably belongs to her but she left her bookcase to her brother.
- It is also hinted that she may have connections with the other characters in the anime, unknown to Hōtarō. Like Fuyumi Irisu, based on their chat as she called Hōtarō as "the idiot".
- It's worth noting that she and Hōtarō are based on Sherlock and Mycroft Holmes with interchanged traits.
- In most aspects, Tomoe is much like Sherlock, being adventurous on top of being knowledgeable in martial arts or self-defense. Hōtarō on the other hand is like Mycroft, someone who didn't indulge in mystery solving (neither being that adventurous) and not being that athletic.
- However, as seen in the series, Hōtarō has exceptional deductive skills, to a similar level of Sherlock. He also has the "sidekick," who is Chitanda. While Tomoe does not show up to offer any signs of deductive reasoning, it is implied that she too is smart, but not as much as Hōtarō, which is a case similar to Mycroft. She also lacks an apparent sidekick.
- This is a very apparent reference to Sherlock Holmes, as the series is referenced a number of times within the whole series.