Tachiarai is a lazy young girl, who sleeps twenty-two hours a day. No one has ever seen her stand on her own two feet. She is also quite flirtatious, telling Kouki Akune that he looks sexy upon seeing him, and was charmed by Misogi Kumagawa. This attraction ultimately propels Tachiarai to stand for the first time, in order to fulfill Kumagawa's request. Despite her usual cheerful air however, Tachiarai does have a darker side, as she reveals a horrifying visage when chastising Kumagawa for taking her too lightly.
Tachiarai has silver colored hair with a pink tint tied in two long pigtails. Due to her constant sleeping, she wears a light pink nightshirt with matching socks and gloves, and a light blue sleeping mask with the phrase "I CANNOT WORK" printed across it in kanji. She is always seen on top of a white body pillow. Ten years later, Tachiarai wears a long-sleeved set of pajamas.
Kumagawa Incident Arc
On August fifteenth, Tokemichi Choujabaru asks Tachiarai for permission to allow Kuudou Hinokage to participate in the Vice-President's Battle. Tachiarai approves. On August twenty-second, Hansode Shiranui uses a commission signed by Tachiarai to allow Kumagawa to participate in the President's Battle. Apparently, it took one month for Shiranui to win Tachiarai's approval.
Kurokami Medaka's Successor Arc
After the second trial, the combined team of Student Council and Candidate members split up at the double stairway with one side labeled "S" and the other, "C." Mogana Kikaijima,Tsugiha Yojirou, and Kiki Kikitsu encounters Myouri Unzen and his version of the third trial. After Kiki stops Kikijima from stripping and appeasing Unzen, she asks what does the "S" stand for which was sleep.
In the center of a very large room, Akune, Kumagawa, Shori Wanizuka, and Ima Takarabe find Tachiarai sleeping warmly. She wakes up and welcomes the tiral goers, complimenting Akune's looks before digging her head into her pillow and brainstorm a challenge for the "S" third trial. Akune comments that Tachiarai is extremely lazy however she is a top-of-the-line genius. Tachiarai explains a little about Ostracism, an ancient election system used by Athens, and says that the "S" third trial will be like that by having each of the members vote who is the most hated and leave the trial without them.
She creates a voting box with the rules taped to it and notices that Akune has found out a way to beat the trial. Akune pretends to fall back to anger Wanizuka while Takarabe and Kumagawa become a team as well. A few of the Election Committee staff were mixed about how this would be helpful to this trial, however Tachiarai finds that Akune's plan to be ingenious. He created two teams of "hate - hate". Wanizuka and Akune will vote for each other and the other two will as well. Thus, they will all be tied, allowing all of them to pass. The team proceeded as planned and all four of them passed the "S" third trial. Afterwards Takarabe picks up Kumagawa's paper that read "Takarabe Ima, because she has a two faced personality" and asks if this was a fabrication.
Tachiarai decided to pick Kumagawa as the member who would join her committee while being the only member who wanted him.
Tachiarai was the announcer for the 100th election and was the first time that the student body saw her standing on her own two feet.
Unknown Shiranui Arc
At the end of the academic year, Tachiarai is among the students present at the Hakoniwa Academy graduation ceremony.
Bouquet Toss To The Future Arc
Tachiarai meant to join the other Student Committee Chairmen to await Medaka Kurokami as a group, to challenge her as part of the 100 Flowers Run. However, Tachiarai overslept, forcing Choujabaru to hold flower number fifty-two and participate in her place. Her message to Medaka is to keep her eyes open.
Ten years later, Tachiarai has developed a cryogenic sleep device, and went into cold sleep right after graduating.
Genius Intellect: Tachiarai is a genius on a level allowing her to enter any of the special classes from 10 to 13, but due to her utter refusal to exert herself, was placed in Class 1. This makes her one of the few Abnormals not in Class 13. In only a brief time, Tachiarai was able to come up with a complex test that seemingly forced the participants to work against each other. She was also the only chairman to see through to Kumagawa's true motives in the game of Bloody Seven.
- Her epithet, Sleeping Lazy, in Japanese is ねむれる怠惰 (Nemurerutaida).
- Tachiarai was voted the fifteenth most popular character in the series in the third popularity poll with 140 votes.