Yuuri Senya

Yuuri Senya (悠里千夜) is a girl who appears in Toaru Kagaku no Railgun Gaiden: Astral Buddy. Supposedly a ghost and suffering from amnesia, she seeks Hokaze Junko's help in finding out who she was.
 * Appears In = Manga
 * Manga Debut = Astral Buddy Chapter 1}}

Appearance
Senya has long dark hair and wears a white dress.

Personality
Playful, mischievous, and excitable, she is always to meet new people. In her first act upon meeting Junko is kissing her. From then on she would cause trouble for Junko whenever she possesses her body, much to her chagrin. In spite of this, her friendly nature and tenacity eventually won Junko over, and she agrees to help her investigate what had happened to her.

Her cheerful personality belies a girl who was full of doubts and fears. Because of her disease, she tried her best to look on the bright side of life, never letting people look down on her and pity her, and not wanting to be treated differently from everyone else.

Background
In her youth she had an incurable disease which was discovered in the spring just before she started elementary school. She was born with weak lungs that puts a burden in one of the four chambers of her hearts, which makes her blood flow worse. Because of this, she needed constant care and saw how people pitied her and treated her differently. Despite this, Senya tries to be cheerful and friendly regardless.

Sometime later her powers were discovered by Clone Dolly, and its third laboratory, Ideal, was created specifically with her as the center of it. As such, she becomes the oldest member of Ideal, and was cared for and looked after by Toomine Kanari, her charming and friendly personality easily winning her over. She would late form a close bond with her, with Toomine knowing exactly how Senya felt without her saying anything.

Much of her time in Ideal was her being in the hospital room, occasionally using her powers to "project" out of her body, though Toomine would sometimes allow other girls to keep her company. Later, after the development of Indian Poker card being able to record the Personal Reality of espers through dreams, the next phase of the experiment began. Senya's Personal Reality would be recorded and then be taught to the other children, and afterwards the result will be sent back to Senya. All of this to create the Ideal esper power, hence the name of the facility. The result would have Senya, the AIM entity that is separate from her body through her power, interfere with the other girls of Ideal's Personal Reality. Toomine herself volunteered to lead the experiment as she knows Senya more than anyone else, and would try to protect the children at all cost. However, the experiment does go awry, Senya's power interferes too much with the other girls' Personal Realities, with their AIM diffusion fields gathering her as the core and possessing a researcher's body. This results in her friends and the adults getting injured or killed, including Toomine, who sacrificed her life to stop her power.

Later, as a "ghost", Yuuri Senya doesn't have any memory of who she is, though she suspects that she is likely the ghost of someone who has died, given her ghostly characteristics and circumstances.

Astral Buddy Arc
After resolving an incident involving a series of pranks previously believed to be the work of a ghost but actually the work of a girl infatuated with Hokaze Junko, Junko talked to the doll involved in the incident, only for the genuine ghost girl emerge from it and greet her, scaring her into unconsciousness. Afterwards, Senya possessed Junko's body and took her back to her room. In her subsequently dream, Junko tried fleeing from her only to wake up in her bed. Senya then appeared in front of Junko, scaring her after which she pointed out to Junko that she was naked much to her dismay. She explained how she brought Junko to her bed and also pointed out that she is late for school.

Through the use of her "ghostly" powers she helps Junko through her adventures as she tries to find a way to discover who she is. She is later involved with a conspiracy involving the Queen of Tokiwadai, Shokuhou Misaki, possessing her body in order to help Junko find her. There she discovers that she, Junko, Misaki, and Misaki's assailant have a connected past.

Abilities
Yuuri Senya's ability is called Astral Buddy (幽体連理)|Yūtai Renri (Asutoraru Badi)|lit. "Ghost Relationship"}} It has the power to remove her AIM Diffusion Field and isolate it as an AIM thought being modeled after her physical physical appearance. In this form, Yuuri Senya displays a number of traits which are commonly associated with ghosts, including, floating in midair, being visible only to certain people and the ability to living beings.

When possessing a person, it creates an avatar made of moisture, which supports the person's physical and mental control. Through this, she greatly enhanced both strength and speed of Hokaze Junko's Rampage Dress, giving her a chance against the Level 5 Sogiita Gunha.

Ideal
Ideal's goal is to create the "ideal" esper power through the recording of an esper's Personal Reality via an Indian Poker card, then teach another with that thereby altering their Personal Reality and recording it again, repeating the process afterwards until the ideal esper power is achieved. Senya's power is at the core of Ideal as it allows her AIM diffusion field to be separate from her body. Her personal reality was the first one to recorded in an Indian Poker card for this experiment and was supposed to be taught on the other children of Ideal.

The experiment was to use the AIM entity separated from Senya by her power. From there, the plan was for her AIM entity was to interfere with the children's personal realities. However, Toomine, the one who was leading the experiment underestimated how great a power Senya had. This resulted in a disastrous experiment where Senya loses control of her powers, and the personal realities that she interfered with gathered around in a mass of AIM diffusion fields and with her AIM entity as the core.

The entity would later possess the body of a scientist that quickly recovered from injury and had a myriad of powers, similar to an AIM Burst. It was only to through Toomine distracting the monster and allowing Mitsuari Ayu to use her powers on it that the possession of the scientist's body was removed and the large aggregation of AIM diffusion fields was dispersed.