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 apart of the Abgrund Family who're known as a household of assassins. She was later hired as a student for Akatsuki Academy to dominate the 62nd Seven Stars Sword Art Festival and served as a temporary first-year in Donrou Academy. Her real name is revealed to be Vier.

After the end of the Seven Stars Sword Art Festival, she returns home and found that her older sister, Ein Abgrund had killed the others including her parents. In order to enact revenge on Ein for perverting their job as assassins, then went to Vermillion Empire where she knew she had gone and soon joined Team Vermillion.

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

Later, during the Vermillion Empire Arc, it is shown that she was dressed in a maid outfit after she infiltrated the Vermillion Palace in order to get to closer to Ein. In the tournament between Team Vermillion and Team Cradleland, she wore a purple dress with the same mask that she wore 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 just sense malice from the petite girl. This is proven even further after trying to kill Shizuku because she couldn't wait for the Seven Stars Sword Art Festival to do it. She can also be both arrogant and a little short-tempered, becoming angered when she feels as if she's being looked down on and wants to kill anyone who dares to do so. Nonetheless, she can also recognize a dangerous situation and will retreat rather than take unnecessary risks for herself.

Despite the above mention traits, Yui is not an evil person deep down, but rather a victim of circumstance. Having been raised by assassins since she was born, Yui grew up needing to be stronger than her "sisters" or she would die. There was a time when Yui has been called kind, surprising and embarrassing her when she realized she was actually worried about someone else.

History
Yui was born to the Head of the Abgrund Family, an organization of assassins under the name of Vier and raised to become one of the deadliest assassins. However, she was most average, performing poorly during training. It was her "older sister", Ein Abgrund who was a prodigy in the household who encouraged Yui not to give up and had even given her the chocolate that she had been given after a successful mission, which amazed Yui from the flavor and served as her motivation to improve. Afterward, Ein gave her three important advice: do not waste too much energy; do not waste too much time, and do not use excessive movements.

Eventually, Yui had begun improving becoming almost as skilled as Ein, however, Ein, on the other hand, started to become more sinister to where she killed her former warmer side replacing it with a crueler one more fitting for an assassin. Yui decided to continue her life as a killer to the end, while also trying to not become like her twisted "older sister".

She was soon hired by Bakuga Tsukikage to be a student of Akatsuki Academy as she worked for Rebellion and later joined Donrou Academy as a temporary student.

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 accepted, 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, overcoming 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 is 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 number 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: Yui's ability as a Blazer. She is able to reflect all her opponent's attacks back at them and would prove dangerous against those with overwhelming power such as Stella Vermillion. Yui can also to reflect the damage she has received and even the power of other things, back to her opponent.

Master Assassin: Having been trained since three-years-old, though she was initially viewed as being average, after some motivation, she was proven to be as skilled as Ein Abgrund who was viewed as a prodigy. As such, it is noted that she has exceptional skills as an assassin.
 * Heatbeat Control: As a professional assassin, Yui learned how to control her heartbeat, letting her control survive in cold locations and gained this skill when she was left in the mountains for a month. She had later taught this to Stella during their time on Edelberg.

Immense Reflexes: Yui has been trained to react to any and all threats since childhood, even in her sleep, which was to be able to wield her Reflector abilities to the fullest. As a result, she can see and react to even the fastest attacks from her opponents, as well as being able to see Ikki's attacks when using Edelweiss' Swordsmanship, in which he stole using Blade Steal.

Immense Endurance: She also possesses amazing endurance, being able to endure a punch in the liver from Stella. Yui was able to withstand the incredible damage from her battle against Ein, continuing to fight despite all the numerous injuries, as well as being able to remain conscious after taking damage from using her own Noble Art, Astral Force.

Keen Intellect: Despite being short-tempered, Yui is highly perceptive, being able to both notice and understand the weaknesses and/or flows in others fighting styles and abilities, as well as how to get around them. She could find out the problems in how Stella fought on the mountain and what she needed to do to fix them. Being raised as an assassin, Yui is able to tell whether someone really has it in them to kill based on what both they says and feels. She even correctly notices that Iris didn't say she wanted to kill her brother out of hatred and she would end up betraying them when the time came.

Device and Equipment
Sweeping Centipede: Yui's Device which takes the form of a chainsaw.
 * Total Reflect (完全反射, Perfect Reflection): Yui's first Noble Art, which lets her reflect her opponent's technique back at them. The effectiveness depends on the power of her opponent's offensive strength, thus growing in proportion to the opponent.
 * Damage Reflect (運命逆転, Fate Reversal): Her second Noble Art, which lets her reflect the damage she receives back to her opponent. Damage Reflect repels the "fate" of receiving damage to the opponent. It, however, requires Yui to receive the damage first, making her vulnerable against attacks that knock her out in one hit.
 * Astral Force (星金槌, Star's Hammer): Her third and strongest Noble Art, in which Yui reflects the power of the Earth's movement itself toward her opponent. However, due to all the massive amount of power behind the attack, it causes enormous damage to everything in the surrounding area including Yui making using the technique very dangerous.

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

Trivia

 * Yui's real name, Vier means "Four" in German, which could mean she's the fourth child as Ein who's the oldest name means "One" in German.