The Miyama-Uguisu Mansion Incident

The Miyama-Uguisu Mansion Incident is a one-shot manga written by Kazue Katō that introduced the main concepts of Ao no Exorcist. Due to the positive reception it received, she began work on the latter. The one-shot was later reprinted in full color in a volume of short stories that Kazue previously created prior to Ao no Exorcist entitled "Time Killers".

Summary
Monaka Miyama-Uguisu (宮山鶯最中) is the forty-seventh head of Miyama Flower Arrangement and is about to turn sixteen in a month's time. Her parents died when she was young, and so she was raised by Oribe (織部), her uncle, who is also a flower artist and the chairman of the Miyama Flower Arrangement Society. She lives together with her uncle in the Miyama-Uguisu Mansion. Oribe has an abusive tendency and slaps Monaka to his content.

While ordering Monaka to create a flower arrangement for the next day's luncheon with the American president before going to bed, Noctis (夜 Yoru) is seen hurrying to the mansion, and intrudes the conversation, inviting himself into the mansion. Showing his identification card as an Exorcist First Class from the Japan Branch of the Knights of the Blue Cross, Noctis claims that Monaka has a "spittle of a demon on her" and orders Oribe to evacuate the room.

It is revealed afterwards that Oribe has been possessed by a demon and is infuriated that Noctis has foiled his plans. Meanwhile, Noctis sprinkles holy water around the room to protect Monaka and tells her to drink some, as he has mixed sleep medicine with it too, before going to bed. As she gets ready for bed, she stares at Noctis and says disbelievingly that he reminds her of someone.

Being unable to sleep, Noctis tells Monaka a bedtime story about a young demon who was an underling for a high-level demon named Pazuzu (パズス Pazusu). The nameless young demon was used to being abused by Pazuzu, but by fate, met a girl who saw him as nothing more than an injured cat and treated his wounds. They started to play together, and grew familiar with each other to the point that the girl gave the young demon the name of Noctis. The girl had also mentioned at the height of their relationship that she wished Noctis was human so that they could talk with each other.

Pazuzu took notice of the girl and decided he wanted to eat her, as it was customary for demons to eat humans when they turned sixteen. Possessing Oribe, Pazuzu created a diversion using Monaka to kill her parents by drowning them while they were attempting to save Monaka from a river. As a result, Monaka has been afraid of water ever since. This event was what inspired Noctis to become an Exorcist.

Upon finishing the story, Noctis hears trouble outside the room with the servants and leaves Monaka, instructing her to head into the lake outside the mansion if she were attacked, as it is now filled with holy water from having placed holy silver into it. Oribe seizes the chance and attacks Monaka, leading her to escape the room towards the lake. As Pazuzu fills Monaka with regret for the death of her parents, Noctis comes just in time to distract Pazuzu and reinvigorate Monaka. In the end, Monaka is able to let go of her past and believe in herself, freeing herself from the chains of Oribe and Pazuzu and allowing Noctis to fully exorcize the demon.

At that moment, Monaka thanks Noctis for his help, but is unable to recognize him. Despite being disappointed, Noctis is content that he was able to help the girl he cared for. The story ends with her starting to make the connection between the exorcist and the cat, as Noctis leaves for his next task.

Differences to Ao no Exorcist

 * The main character was not Rin but a similar character named Yoru (夜) in Japanese and Noctis or Night in English. He is a Demon similar to Kuro who has taken human form.
 * The main villain is named Pazusu (パズス) in Japanese and Pazuzu in English, who shares some similarities to Satan.
 * The secondary character, Monaka Miyama-Uguisu, is similar to Shiemi.
 * Oribe, Monaka's uncle and guardian, is possessed by Pazuzu, similar to how Shiro Fujimoto became possessed by Satan.