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. Later she wore a maiden outfit, as she infiltrated the Vermillion Palace.

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, being a member of the Abgrund Family, a famous family of assassins. During her childhood she was forced to under go great amounts of harsh training. For example, when Yui was a child she was left alone with her dying sister on a mountain for an entire month.

Eventually she got hired by Bakuga Tsukikage to be a "student" of her Akatsuki Academy, as she already worked for Rebellion.

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.

Responsibilities as a Royalty
After Lunaeyes' announcement, Ein is ordered to retreat by All-Goal. As Ein withdraws, Yui questions if she is cowardly running away. Ein though says that if Yui thinks she has a chance against her, then she should chase after Ein. Yui though doesn't move, letting Ein escape.

It's later decided that Yui will represent Vermillion in the upcoming tournament, along with Stella, Ikki, Iris Ascarid and Nene Saikyo. Yui also decides to accompany Stella, Ikki and Ascarid to Estonia, where Stella wants to challenge Edelweiss.

White Summit
Once the team arrive to a small village near Edelberg, the mountain where Edelweiss lives, they almost get into a fight with a mercenary group called Murder Entry. The conflict is interrupted by Yoshikazu Kiba and Ranbalt Raab, who are also there to challenge Edelweiss. The group though decides to head for Edelweiss, instead of staying with Kiba and Raab. Stella flies up to the mountain, carrying Ikki and Ascarid, while Yui is forced to use her ability to continually bounce through the air.

When they arrive up on the mountain, they see a cabin where Edelweiss lives. Stella attempts to fight Edelweiss, but she is unable to even approach her, because of Edelweiss' Desperado aura. After realizing she can't defeat Edelweiss, Stella asks Edelweiss to train her. Edelweiss tells Stella, that if she guards her home for six days, Edelweiss will train her for one day. Ascarid warns Stella that if she agrees, Edelweiss' Noble Art, Rule of Grace will make her unable to back out. Despite the warnings, Stella agrees to Edelweiss' suggestion. Yui and Ascarid also are forced to swear that they won't help Stella.

Intense Cold Trial
Yui and Ascarid later watch, while Stella battles with dozens of Blazers. In the end, when only four Blazers remain, Stella starts to get sloppy, not even realizing her opponents' abilities. Thanks to Yui's advice though, Stella manages to defeat the Blazers. After Stella's fights are over, Yui scolds her, for her careless and wasteful fighting style. Yui then goes on to teach Stella to better her fighting, telling there are three things in Stella's fighting style, that are useless. The first one is unnecessary wasting of calories, which power Stella's abilities. Yui teaches Stella how to control her heartbeat, in order to avoid needles wasting of her power on warming herself up. The second one is the amount of time Stella wastes on fighting enemies, who are much weaker than her. Yui teaches Stella ways to take care of enemies quicker. The third one is the amount of openings Stella wastes during fighting. After learning from Yui, Stella has a far easier time taking care of bigger groups of enemies, easily holding the enemies of for five days.

On the night of the fifth day, Yui, Ascarid and Stella relax in a bath. Yui also tells about her past in the Abgrund Family, and how she wants to kill Ein, to restore the credibility of the Abgrund Family.

The Assassin from Shinryuu Temple
On the sixth and final day, Stella is challenged by Fu Xiaoli, one of the Four Sages of Shinryuu Temple. Despite Xiaoli being a Desperado, Stella manages to beat her, after her soul evolves.

After beating Xiaoli, Edelweiss allows Stella to challenge her, but Stella declines, believing she is already strong enough, and doesn't need to fight with Edelweiss. Stella and the others than return to Vermillion, where the tournament will be held shortly.

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 of the Abgrund Family..
 * Heartbeat Control: As a professional assassin, Yui is capable of controlling her own heartbeat, allowing her to survive in very cold places. She gained this skill when she was left on the mountains for a month.

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 (完全反射, Kanzen Hansha): 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.