Yui dons futuristic attire characteristic of the time period of the AI Uprising. She has brown, medium length hair and brown eyes. Uniquely, she uses her grandfather's piano key as a hair accessory. She is often seen in a wheelchair.
Yui is seen as a great leader with a good heart, as she was the leader of the Moderate Faction of Toak, which holds her grandfather's wish for human-AI coexistence. Along with Elizabeth, she is seen going in the frontlines, even though she was in wheelchair, and helping on commanding Toak members on the Arayashiki tower assault.
She considers Vivy and Elizabeth her friends, and discourages conflict before knowing the reason first. Even when she was about to die, she still asks a Rogue AI his name, displaying kindness on enemies.
Yui was the leader of the Moderate Faction of Toak, where she believes that Humans and AIs can co-exist with each other. She also made Elizabeth (in a new body, but data memory from before the sunrise incident) her Life Care AI.
On the second timeline, she met Vivy after she saves Dr. Matsumoto and saves them from the Rogue AIs. She recognizes Vivy when they met, as her grandfather Yugo Kakitani went rouge just to find her, though she formed the Moderate Faction of Toak from her grandfather's wishes. As Dr. Matsumoto informed them that the Singularity Project failed, as the end of the timeline is still the war between humans and AIs, she informed them to regroup and plan for their next action. As Vivy recognizes Elizabeth, Yui shows her grandfather's last wish, making Elizabeth's mission more firm. When Vivy realizes that their main enemy was the Archive itself who went rogue, Yui regroups the remaining Toak members for a final assault on Arayashiki tower, the heart of the Archive itself. As they arrived on the tower via a speedboat, Yui remains on board to support them while traversing the tower, and having a Matsumoto cube as assistant while Elizabeth goes with Vivy. While traversing, the tower structure suddenly shifted unexpectedly, and while her and Matsumoto working on the security program to reverse the building shift, they we're ambushed by rogue AIs, and was fatally injured. As her final wish for Elizabeth, she requested as a friend to complete the job in the end, before getting killed.
As Vivy went back on time for the third time, she saves all the Toak members instead of saving Dr. Matsumoto, which made Yui mournful as they lost a good friend and scientist. They still proceed the same strategy as the last timeline, but this time, Vivy went to NioLand Main Stage, while most of Toak members, including Yui, went to assault the tower.
Yui is last seen listening to Vivy's last song as she was helping the assault team and successfully averting the satellite fall.
- The name Yui is written in katakana (ユイ), which has no special meaning. However, when written in kanji (結衣, 優衣 or 結), it could possibly mean:
- 結衣 - "tie, bind" (結) (yu) and "clothing, garment" (衣) (i).
- 優衣 - "excellence, superiority, gentleness" (優) (yu) and "clothing, garment" (衣) (i).
- 結 - "tie, bind".
- Yui's surname Kakitani means "hedge, fence" (垣) (kaki) and "valley" (谷) (tani).