Yui Tatara

Yui Tatara is a member of Rebellion who is hired to serve as a student for Akatsuki Academy. Her real name is revealed to be Vier. She is also part of the Abgrund family of assassins.

Appearance
Yui is a petite girl with black hair and purple eyes. She is dressed in a gray winter clothing, a scarf and a gray hat with a skull design.

Personality
Yui is shown to be extremely ruthless and sadistic, wanting to kill someone just to vent off her stress. In fact, Ikki, upon his first encounter, mentioned that he could only sensed malice from the petite girl. This is proven even further after trying to kill Shizuku because she couldn't wait for the festival to do it.

Yui is also arrogant and short-tempered. She will become angry when feels she being looked down up and want to kill anyone who dares to do so. Nonetheless, she can recognize a dangerous situation and will retreat rather than take an unnecessary risk for herself.

History
Yui has been known to be killing people since she was young before she got hired by Bakuga Tsukikage to be a "student" of her Akatsuki Academy.

Akatsuki Strikes
Yui was first seen, along with her fellow Akatsuki Academy members, going toward Hagun Academy. She was the first to try and attack, taking on both Ayase and Yuuri and had some how defeated them both using an unknown ability, named Total Reflect.

Yui was then seen, with her fellow Akatsuki members, about to clash with the other Hagun Academy students, however, Akatsuki academy was betrayed by Nagi who had concealed their movements and allowed the Hagun students to defeat them. However, Yui and the other's were revealed to be hiding in a different space.

After Ikki, Shizuku, Stella and the Hagure sisters had left the scene, Yui and the other Akatsuki members had defeated the student council members, easily, and had decided to continue their onslaught, by going after Stella and the Hagure sisters.

They had eventually found the sisters and Stella but their efforts to hurt them were impeded by Nene, who was able to single handily stop Yui, and most of the Akatsuki members, besides Ouma, with her gravity ability. Yui and the others had decided to retreat, as they had completed what they wanted to do.

Party Crashers
Yui is seen, after her other fellow members left the party, confronting Ikki and his sister Shizuku. She offers Shizuku a piece of meat, which she accept, however it is slapped out Ikki's hand who reveals that it contained blades in it, which would have possibly killed Shizuku upon her first bite. Yui then laughs, explaining the reason did it was that she couldn't wait to kill.

Yui then activates her chainsaw device causing Ikki to activate his device and a fight nearly breaks out until it is interrupted by the current Seven Stars Sword King, Yudai Moroboshi. Yui does not attack Yuudai as she understands his strength but is eventually forced to leave after more representatives came, ready to defend Ikki, including Kuraudo Kurashiki.

One vs Four
In the preliminary round of the festival, Yui accepts Stella's challenge to fight against her, alongside Rinna, Hiraga, and Mikoto. Yui had already finished her preliminary match, but still decides to take on Stella. The match begins and Yui attacks Stella, first. Stella is, however, able to easily dodge her attacks. Yui is eventually taken down by Stella's pure power, however, over coming both her device and her reflector ability. Yui is left too injured to continue the tournament causing her to drop out of it.

Kardia Urban Warfare
After the Seven Stars Sword Art Festival Yui returns home, only to find her entire family murdered by her sister Ein. Yui then decides to avenge her family by killing Ein. She infiltrates Vermillion, as a maid, because she believes Ein will appear there.

When Ein shows up at the Vermillion castle, as Yui expected, she almost kills the Emperor of Vermillion, Sirius Vermillion. Yui though saves him with her Total Reflect. She then confronts Ein, but before they can start their fight, they are interrupted by Lunaeyes' speech.

Abilities
Reflector: The ability to reflect all their opponents’ attacks back at them; their expertise lay in the way their ability would increase in power the stronger the attacks received were.

Master Assassin: Having been trained since 3 years old, Yui is noted to be an exceptional assassin for Rebellion.

Immense Reflex: Yui been trained to react to any and all threats since childhood for the sake of using her Reflector to it fullest. As a result, she can see and react to even the fastest of attacks from any opponent. She was even capable of seeing Ikki Kurogane while he was using Edelweiss' high speed movement.

Device
Sweeping Centipede: Yui's Device which takes the form of a chainsaw.
 * Total Reflect: Yui's Noble Art which allows her to reflect her opponent's technique back to the opponent. The ability depends on the power of her opponent's offensive strength, growing in proportion to the opponent.

Trivia

 * Yui's real name, Vier means four in German.