Mirai Kimizuki

Mirai Kimizuki 「君月 未来, Kimizuki Mirai」is the younger sister of Shihō Kimizuki who was infected with the Apocalypse Virus despite being a little girl and was being treated in a hospital. Shihō joined the Japanese Imperial Demon Army in order to gain access to the military resources that were needed to cure her as the virus could not be treated in civilian hospitals.

Human
Mirai is young girl with long reddish brown hair and reddish pink eyes. She has purple curse marks from the Apocalypse Virus on the left side of her face that fluctuate with her current state of welfare. She is most often shown wearing a hospital gown.

Seraph
Mirai no longer has the curse marks on her face, her eyes are dark with peculiar markings on her irises, and sports two pairs of large feathered wings. When Abaddon possess her, the demon manifests in a giant dark figure with many wings and an irregular form behind her. She is still in the hospital gown and is barefooted.

Anime:  The curse marks remain and increase.

History
Before the apocalypse, Mirai lived a normal life with her parents and older brother, Shihō Kimizuki. At some point, she and Shihō were experimented on by the Hyakuya Sect to become weapons of mass destruction known as the "Seraph Of The End" to destroy the world. Guren knew of this beforehand, thanks to intel provided by Mahiru eight years ago. That tragic night, the Apocalypse Virus got released on Christmas 2012, killing her parents and becoming infected herself, even though she was a little girl, forcing her brother to take care of her.

Three years later, in 2015, Mirai fled with Shihō, though he had to carry her due to the Virus making her too weak to move, thus made things difficult for her older brother over time.

Second Shibuya High Arc
Mirai first appears in Chapter 4 when Shihō and Yu (due to being handcuffed to each other) left in the middle of their exam to come see her in the infirmary. The healthcare provider informs Kimizuki that she pulled out of danger this time, but the treatment available to civilians is not enough. Kimizuki tells him that soon he'll be in the Moon Demon Company and will be able transfer her to an army hospital.

After they leave, Kimizuki blames Yu for letting his grades drops which affects in ability to enter the Moon Demon Company and because of that, Mirai would not be able to be saved. Yu tells him to not give up on her and Kimizuki soon thanks Yu for letting him be able to be there with her when it counted.

Nagoya Arc
Mirai reappears again much later in Chapter 37. In a Demon Army helicopter, she screams about pain while scientists watch and observe her.

One orders an increase in dosage which another scientist tries to protest. The one from before says that it's fine as Kureto Hīragi gave him the order to increase the medicine dosage. Mirai begs for them to just kill her before receiving an injection which causes her immense agony to the point of pleading for her big brother while also causing the curse marks on her face to disappear. Two pairs of feathered wings then erupt from her back as her screaming stops. She is seen next in chapter 40 where she erupts from a Moon Demon Company van and floats in the sky with four large feathered wings and a single long trumpet before her. She curses the sinners and says ruin will fall down upon them. Kimizuki recognizes her, shocked and horrified.

Chains erupt from the remaining van and pierce through Mirai, making her scream in pain as a magical circle appears behind her. While Kimizuki is enraged, Guren stabs him from behind and tells him to hold still because they have not yet offered enough blood to control Mirai.

Up above, a black aura appears from Mirai's back. The soldiers announce that the fifth trumpet is transforming into an angel of destruction to smite mankind. Although they have not offered enough sacrifices yet, they have achieved the proper state, and Kureto's subordinates claim they are successfully controlling a Seraph of the End. He orders Aoi to feed the rest of the sacrifices to the demon Abaddon to stabilize it. The demon's power rushes out and consumes nearby vampire and human soldiers while they look up in horror.

The demon becomes fully stabilizes but a few moments later, Yu awakens in his Seraph form and destroys Abaddon's manifestation.

Post-Nagoya Arc
When Kureto and Guren invade Sanguinem, Mirai is with them and the army in her Seraph form.

Osaka Arc
When Kureto returns to Shibuya, he tells Aoi to prepare the seraph to take over. She is then used to attack Shibuya to aid Kureto's coup.

Experimentation
Seraph of the End: As the 5th Trumpet, Mirai possesses great power that can be felt when she blew into her trumpet, and while the extent of her power has yet to be shown, she seems to be powerful enough to fully invade a vampire city. Her trumpet also served as a way for Kureto to summon the demon, Abaddon.

Trivia

 * Mirai 「未来」 means "Future."
 * Kimizuki 「君月」 means "Your Moon."
 * Despite being younger than 13, she was infected by the same virus that killed those 13 and older during the Apocalypse. Shihō took care of her until he entered the safety of the JIDA.
 * Even though she is shown covered in ofuda in the manga, the ofuda do not appear in the anime.

Quotes

 * "AAAAAAAAAAAAHHH!!! Stop!! It hurts!! Please!! I can't take any more!! The pain...!! I hurt all over!! Stop... Kill me... Please, just kill me... Eep... AAAAAAH!!! Help me!! Big brother!! Big brother!! Help me!!! AAAAAAAAAAAAAAAAH!!!!"--Mirai to Kureto's scientists, Chapter 37, "Monsters & Family"
 * "Wretched sinners. Ruin shalt fall down upon thee."--Mirai in her seraph form, Chapter 40, "Trumpet of the Apocalypse"