Mirai Kimizuki

Mirai Kimizuki 「君月 未来, Kimizuki Mirai」 is the younger sister of Shihō Kimizuki. She was infected with the Apocalypse Virus despite being a child 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 be treated in civilian hospitals.

History
She was one of the children that was being experimented on by the Hyakuya Sect to create a weapon of mass destruction, as part of the 'Seraph of the End' plan in destroying the world. Guren knew of this beforehand, thanks to intel provided by Mahiru eight years ago.

She got infected with the Apocalypse Virus despite being younger than thirteen. Kimizuki took care of her, even though they lived in the outside world with monsters. She wanted Kimizuki to kill her, but he refused and they eventually entered Shibuya. By joining the army, Shihō got the resources and money needed for her care.

Second Shibuya High Arc
Mirai first appears in chapter 4. During their test, someone arrives and informs Kimizuki that she is in critical condition. Kimizuki asks to continue the exams, but Yu convinces him that the test isn't nearly as important as being there for her.

After much protest, Kimizuki finally decides to go see Mirai in the infirmary (with Yu coming with him because they are still handcuffed together). 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. Kimizuki 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, Mirai screams about pain while scientists watch and observe her.

One orders an increase in dosage. Though another scientist tries to protest, the one from before says that it's fine as Kureto Hīragi gave him the order to increase the medicines dosage. Mirai begs for them to just kill her. She receives an injection and begins screaming for help from her big brother as the markings from the Apocalypse Virus on her face disappear. Two pairs of feathered wings erupt from her back as her screaming stops.

She is seen next in chapter 40. She erupts from the van and floats in the sky. She has four large, feathered wings and a single long trumpet before her. Her eyes are dark with peculiar markings on her irises. 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. He 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 in her Seraph form.

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"