Yuuka

Yuuka is a physician of Messiah and secretly, a spy working for the 'Alliance'.

Personality
Yuuka was once a happy woman until the death of her husband due to Arcadia's fall radically changed her. In present, she was now a wicked woman who solely desired vengeance upon those who wronged her. Yuka would manipulate, undermine and murder anyone to accomplish her goals. While undercover as Messiah's physician, she presented herself as a kind and concern woman.

Background
Yuuka lived in Arcadia with her husband who was one of Arcadia's scientists that produced Japan's First Knight. Tragedy struck however when the First Knight went on a rampage due to Akira Nishino's experimentations resulting in countless deaths, including Yuuka's husband. Yuuka held her husband as he died.

Years later, Yuuka joined a group called The Alliance who tasked her to infiltrate Messiah and sabotaged the group alongside their mole, Yuudai Saejima so that they could claim their base as their own. Having successfully infiltrated Messiah, Yuka learned to her outrage that not only Akira Nishino was still conducting his experiments but Akane Nishino, the First Knight had forgotten what happened to Arcadia. Yuka spent her time getting close to Akane all the while secretly planning her revenge against her.

Revenge against the Nishino Family
When Akane brought back two survivors, a boy named Minoru and a girl named Natsume from a Magical Beasts nest, Yuuka did her best in taking care of them them. She would explained to Minoru about the Magical Beasts and their nests, believing he had lost much of his memories. While tending to Natsume, Yuuka found her watch was missing but concluded that it must have fallen off.

After a Magical Beast stampede was stopped by the Black Knight, Yuuka eliminated Yuudai Saejima per the Alliance's orders but the woman took advantage of the man's death to further her own goals of revenge. She cut Yuudai to pieces with a chainsaw and framed the death as Akane's doing. When Yuuka provided Akane the "evidence", Akane went into a nervous breakdown until Yuuka sedated her and took her away to tend to her. Unbeknownst to Messiah, Yuuka actually kidnapped Akane.

Yuuka meet up with Minoru who revealed proof of Akane's innocence. Worried that the boy will discover the truth, Yuuka secretly ordered two hitmen to take Minoru out. During another Magical Beast stampede, Yuka took the chance to find Akira Nishino and killed him along with Natsume. She took the Brute's head that Akira wanted to use. Yuka coldly stated down at Akira, telling him that she hoped his death will hurt.

Death
Having escaped Messiah, Yuuka returned to the spot where she had kept the unconscious Akane Nishino. Suddenly, she was confronted by Minoru who deduced her as the Alliance spy. Yuuka explained her past and intentions to Minoru before injecting Akane with the Brute's blood. With Akane now transformed into her First Knight state, Yuuka allowed herself to be killed at the First Knight's hands. Despite nearing her end, Yuuka hugged the First Knight before laughing, having finally gotten her revenge.

Powers & Abilities
Yuuka was just an ordinary human with no magical abilities whatsoever. However, being Messiah's physician, it can be assumed she knew medicine. She was also a skilled spy, capable of infiltration the human refuge without anyone deducing her true identity until it was too late.

Unnamed Husband
Even after the invasion by magical beasts, Yuuka lived happily with her husband until the birth of the First Knight where the knight lost control and destroyed Arcadia taking her husband's alongside it. His death would result in Yuuka changing for the worse as she swore allegiance to the Alliance and becoming hellbent on avenging his death.

Akane Nishino
Yuuka is aware of her identify as the First Knight and despised her for taking her husband's life. As a spy, she would put on a facade of a nurse and would help her alongside other habitants of the Messiah. When the opportunity shows, she would try to out Akane for murdering Yuudai and eventually get revenge when she made her recall her memories of the First Knight thus causing her to go berserk and contemplate suicide.

Akira Nishino
Brother of Akane and researcher for the Messiah, Yuuka is also aware that he was responsible for the destruction of Arcadia and the death of her husband and held it against him. As a spy, she and Akira would work together when it comes to research and medical care. She would eventually have revenge on him when he tried to run off with Mordred's head.

Yuudai Saejima
Fellow co-conspirator for the Alliance, Yuuka at one point bribe Yuudai to their side as their mole and work together as spies. However after a magical beast stampee, she would betray and kill him when she got what she needed. She would also use his death as an attempt to out Akane as a murderer.

Haitani
Commanding Knight for the Messiah, not much is known of their interaction aside from performing tasks ordered by Haitani. Haitani was unaware that Yuuka was the spy for the Alliance during Shadow and Beta's time on Earth.

Minoru / The Black Knight
Initially, Yuuka did not think much of Minoru when he first woke up in the infirmary. However, much of it changed when he found out more details regarding Yuudai's murder where she tried to put a hit on him after he elaborated on the evidence. As the Black Knight/Shadow, Yuuka would not get the last laugh in her quest of vengeance when Shadow saved Akane from going berserk and helped her accept what she's done with a new purpose in life.

Natsume
Yuuka was wary of Natsume, believing that she is awakened like Akane. Initially she was going look after her before Akira cut her off and decided to run experiments on her. When Akira tried to experiment on Natsume with Mordred's head, Yuuka stopped him by gunning down the two. However, no one but Shadow is aware that Natsume can survive gun shots thus only leaving her alive from the ordeal.

Trivia

 * Yuuka can be considered the second female villain, the first being Reina Oriana.