Yui (ユイ Yui) was the adoptive daughter of Prime Minister Honest as well as the vice-leader of Wild Hunt. Due to her status and the crimes that she committed, she was the tertiary antagonist of the series, alongside Syura her adoptive brother. In truth, she is the younger sister of Akame and the twin sister of Kurome.
Being the twin sister of Kurome and younger sister of Akame, she looks exactly like Kurome, and very similar appearance-wise to Akame. She has short black hair in a twintail style and black eyes. She wears a black sailor uniform and wears a red belt that has a red side skirt cover like Akame. She also wears red gauntlets like Akame. She keeps her Teigu with her at all times. She wears it with long black socks and black shoes.
Although she shares some traits with her sisters, such as her social awkwardness and a big appetite, Yui is far more innocent and optimistic than both of them, naively unaware of the world outside of her adoptive father's house and labels Syura's enemies as her enemies, slaying them down without mercy. This extreme loyalty seems to extend to allies of Syura s as she respected her superiors and openly defended Boda from Mine on Syura`s orders without further prompting from him. Due to these personality traits, she was considered one of the strongest assassins in the Elite Seven. As a result, she was transferred from the Elite Seven to the Elite Seven Jaegers, originally formed by members of the Elite Seven and chosen to be their leader. She seemed to find Syura and her adoptive father's actions perfectly okay and even enjoyed them on some rare occasions, when asked that question by Syura himself. While she was a naïve person at heart, she had a much darker and aloof side where she was cold and ruthless in battle, usually ending the lives of her enemies as one would expect from a seasoned assassin. Due to being more innately talented than Kurome, she was deemed a more useful person and was raised by the Prime Minister, per the Emperor's orders who had heard of her innate abilities. She was also extremely concerned, or rather obsessed, with pleasing her older adoptive brother, Syura and would become emotionally unstable whenever she felt she was in danger of losing his approval. As Lubbock stated, she was a naïve person who blindly followed the orders of a madman and that he would have raised her better, a statement that almost caused Yui to kill him on the spot.
Shortly after the formation of the Jaegers, Yui returned to the Capital alongside Syura after traveling for a while. Unbeknownst to the authorities, they unleashed a horde of new Danger Beasts, created by Dr. Stylish, resulting in them terrorizing peaceful villages around the Capital.
Syura observed the battle between the new Danger Beasts and Seryu and noted to Yui that the Jaegers were very powerful, but not as strong as the group he intended to put together, deciding to let them "play" with his "toys" in the meantime. Yui scoffed at that, and said the Jaegers were more of a threat now and that they should deal with them now, only to be reprimanded by Syura who beckoned her to watch longer.
Later, they found Esdeath and Tatsumi together at the summit of Mt. Fake, and after being noticed while hiding, he noted to Yui that it was too early to let the Prime Minister know about his return. He then used his Teigu, Shambhala, to teleport them to a remote island in the far south, where they were attacked by a giant Danger Beast very similar to Dr. Stylish's ultimate form.
When Tatsumi and Lubbock attempted to infiltrate the palace with help of the Revolutionary spies, Syura, Budo, and Yui intercept the two Night Raid members with Syura already destroyed the Revolutionary hideout and killed its occupants much to Tatsumi's disgust.
After Budo went off to fight Tatsumi now clad in his Incursio, Syura then battles Lubbock, telling Wild Hunt along with Yui not to interfere in their fight as he apparently had the advantage over Lubbock thanks to his Shambhala. Yui calmly sits on the ground, and shines her sword being surrounded by the members of Wild Hunt who keep up conversation with her, as they watch Syura battling Lubbock. But in his arrogance, Syura was caught in Lubbock's deadly threads that were set up during his running around the palace earlier, resulting in his arm being sliced off in great pain. Yui comments on his arrogance before sliding her sword back into the sheath, a content look on her face.
In one final attempt to kill the Night Raid member, Syura sends Lubbock in Shambhala's space dimension to die there but ultimately he was killed by Lubbock in his final breath before falling to his death by Syura's guards. Yui then buried her older adoptive brother and went with Budo to file a report to her adoptive father, who decided to assign her to be Budo`s partner for the time-being until he could find her a more "sufficient partner" to fight with. In the aftermath, Budo came to inform Honest of his son's death at the hands of Lubbock but was disgusted of the prime minister's lack of sympathy for his kin and the prime minister`s congratulations of Yui.
At Tatsumi`s execution, Yui was present where they noticed Honest's quick leave after the child emperor's speech before a large crowd gathered to witness a Night Raid member's execution. When Night Raid arrived to rescue Tatsumi, Yui and Budo were quick to battle them. During the battle, Budo proves to be a hard adversity, incapacitating Leone early as he focuses all his attack on Mine, fatally injuring her in the process. Budo was eventually killed when Mine, near her final breath, shoots out a powerful blast from Pumpkin that completely incinerates Budo. Yui was later killed by Akame during their ensuring duel, and openly apologized for her actions before succumbing to her wounds and dying of blood-loss.
Events of Akame ga Kill Zero!
Along with her sisters Akame and Kurome, Yui was among a group of children sold to the Empire who were forced to participate in a survival exam in a forest full of danger beasts. After the sisters survived by working together, they were drafted into different assassination divisions (because of their superiors reluctance to keep sisters in one group). As the stronger of the twin sisters, Yui is accepted into Gozuki's new Assassin squad, the Elite Seven, and is trained in the arts of espionage and combat, becoming a highly effective killer. The seven were given Shingu, weapons in which the Emperor 400 years ago created in order to surpass the Teigu but failed to surpass them. Yui was given Dark Blade, a Shingu where wounds inflicted by the katana can be healed, but the poison from the blade remains in the blood-stream forever causing the person pain for the rest of their days. After a long unspecified period of time training, the seven were given their first mission.
Before the Events of Akame ga Kill!
At some point, after many of her comrades' deaths, Yui was assigned to the assassination group known as the Elite Seven Jaegers becoming their leader and completing many successful missions that way. Pleased with her successes, she is reassigned to Wild Hunt and is adopted by the Prime Minister becoming his adoptive daughter.