Yui Tatara

"I was raised in a clan of killers who served Rebellion from generation to generation. Tried and proven hitmen!"

- 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. During the tournament between Team Vermillion and Team Cradleland, she wore a purple dress with a mask, that she also used during the Seven Stars Sword Art Festival party.

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 sense 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 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 was born to the head of the Abgrund Family, an organization of assassins. She was given the name Vier and raised to become one of the deadly assassins of the Abgrund Family. However, Yui was at most average, performing poorly during the training. Her "sister" Ein, who was a prodigy in the Abgrund Family, however, encouraged her to not give up and even gave her some of the chocolate she had been given for a successful mission. Yui was amazed by the flavor of the chocolate, which served as her motivation to improve. Ein also gave her three important advice; to not waste too much energy, to not waste too much time, and to not use excessive movement. Eventually, Yui began to improve, becoming almost as skilled as Ein. Ein, however, began to develop into a far sinister direction, eventually killing her former warm personality and replacing it with a cruel one, more fit for an assassin. Yui, on the other hand, decided to continue her life as a killer to the end, while also trying to not become like her twisted "sister".

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 somehow 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 Or-Gaule. 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 arrives at 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 and Stella form an agreement that Stella will guard Edelweiss' home for six days, with Edelweiss promising to train her afterward, The agreement is also enforced by Edelweiss' Noble Art, Rule of Grace, that forces the parties to keep the agreement. 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 needlessly wasting 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 excessive movements Stella uses during the fighting. These are the same advice as Yui was given by her "sister". After learning from Yui, Stella has a far easier time taking care of bigger groups of enemies, easily holding the enemies off 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 then return to Vermillion, where the tournament will be held shortly.

Imperial War Outbreak
On the day of the tournament, Team Vermillion and Team Cradleland face each other in Lucier, the capital of Cradleland, that will serve as the battlefield for the tournament. As the tournament begins, Naseem Al-Salem creates a sandstorm that throws all of the participants into the air. Yui is able to land safely, using her Total Reflect and goes to search for Ein. She eventually finds Ein in the rose garden of the imperial castle.

The Unturning Assassin
The battle between the two quickly begins, with Yui furiously attacking Ein. Ein, however, manages to hold her of using numerous different plants, forcing Yui to evade and destroy. Ein manages to eventually catch Yui, showing a vine down her throat causing immense pain and damage to her. Ein tells Yui that if she surrenders now, she will let her live and continue her life, letting her go for a moment to answer. However, Yui instead uses her Noble Art, Damage Reflection to reflect all of the damage she has received back to Ein, causing her multiple injuries. Despite the injuries, Ein isn't defeated, activating her most powerful Noble Art, Yggdrasil Abyss. The Noble Art creates a large tree from Ein, that then empowers all her abilities and allows her to create monsters from the surrounding plants. These enhanced plants and monsters overpower Yui, causing her to fall to the ground. While down, Yui remembers her past, when Ein was far warmer and nicer, These memories encourage Yui to continue the battle, rising up. Yui then attacks Ein again, managing to overcome Ein's plants due to the three advice that Ein gave her in the pats and the stimulant Angel Dust, that enhances her abilities. However, Yui is extremely injured. When she finally reaches Ein, Yui thanks her before utilizing her third Noble Art, Astral Force, that reflects the power of the Erath's movement itself to the surrounding area. This causes enormous damage to everything in the area, including Yui, and destroys Ein's Yggdrasil Abyss and likely defeats Ein herself. Yui, however, is able to remain conscious, refusing to die due to Stella having promised her a grand funeral, that would be a disgrace to an assassin.

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. Yui is also able to reflect the damage she has received and even the power of other things, back to her enemy

Master Assassin: Having been trained since 3 years old, Yui is noted to be an exceptional assassin of the Abgrund Family. Immense Endurance: Yui has shown possessing amazing endurance. She was capable of withstanding an incredible amount of damage during her battle with Ein, continuing to fight despite the numerous injuries. She was even able to remain conscious after taking the damage of her Noble Art, Astral Force. This endurance is caused by her childhood training.
 * 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.

Angel Dust: A special stimulant given to the assassins of the Abgrund Family. It increases the user's physical abilities, like strength, speed, and endurance, greatly, allowing them to exceed their previous limitations.

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.
 * Damage Reflect (lit. "Fate Reversal"): Yui's secondary Noble Art, that allows her to reflect the damage she receives back to her enemy. The ability repels the "fate" of receiving damage to the opponent. However, the ability requires Yui to receive the damage first, making her vulnerable against attacks that knock her out in one hit.
 * Astral Force (lit. "Star's Hammer"): Yui reflects the power of the Earth's movement itself towards her opponent. Due to the massive amount of power behind the attack, it causes enormous damage to everything in the surrounding area. This includes Yui, herself, which makes using the technique very dangerous.

Trivia

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