Caren Hortensia

Caren Ortensia is a member of the Church and works as a priestess. She was seen helping the Burial Agency due to her ability in a short story of the Character Material, although she is not stated to be a member of it - she isn't even an Executor. She is angelic and forgiving to the point that she believes her only purpose in life is to help others, even if she is hurt in the process. She never blames the person who has hurt her and blames the act on demons who caused the person to commit the sin. Whenever she is at fault, she apologizes to God rather than the person.

One of Caren’s most unexpected personality quirks is her love for teasing and exploiting others’ emotional vulnerability. Upon discovery of an individual’s weak spot, she enjoys bringing it out to light in front of all the other characters. While this antagonizes many characters and causes much outrage and humiliation, she remains calm: once, when Avenger retorted in response to her, she squeezed his face without a change in expression. Oddly enough, in spite of being a priestess, she has deliberately chosen to wear a revealing costume with no skirt to both increase her mobility and to seduce men.

Past
Caren is a rather mysterious figure and little is known about her past. Her weak mother died soon after giving birth to her, while her father had left her mother before Caren was born. Her mother was actually the wife of Kotomine Kirei, who ended up committing suicide in hopes that Kirei would find compassion within himself again. This implies that Kirei is Caren's Father ; the manga based on Unlimited Blade Works mentions that Kotomine has a daughter, though Kotomine comments his daughter has been living with her mother's relatives since his wife died. As it is, Caren's surname is the maiden name of Kirei's wife.

Left an orphan, she eventually became a priestess in the Church. In the beginning of Hollow ataraxia, she has been sent to Fuyuki City by the Church to replace Kotomine Kirei, who has died, and report the aftermath of the 5th Holy Grail War. Instead, she ends up becoming the mediator of the war because of the four day time loop which goes back to the fourth day of the 5th War.

Abilities
As a member of the Church, Caren assists in exorcisms. Her body itself possesses an unusual property which is known as Masochistic pneumatic automatism diathesis (被虐霊媒体質); if there is a person nearby who has been possessed by a demon, she will experience the same pain as the person possessed. As a result, exorcists employ her to act as a radar of sorts to find demons. This ability can't be gained from training since it's purely genetic, which is why people born with these traits are valued by the Church as demon detectors.

This ability is most evident whenever she comes in proximity to the Servant Avenger as her body sprouts grotesque spikes. As a result of this power, she is covered in bandages. She also experiences physical pain whenever people in proximity do evil things.

In battle, she uses a red cloth of Magdala, which has the power to forcefully bind men. However, while the man is bound, no harmful physical attacks can be made against him or Magdala will be rendered useless.

Noli me Tangere: Caren's invocation of the Magdala cloth, which is a reference to the Lady Magdalene from the Bible's Book of John.

Role
Caren serves an objective mediator in the false Holy Grail War created by Avenger and Bazett Fraga McRemitz and, in the place of the absent Kotomine Kirei, as the Master of Lancer and Gilgamesh. Because she was not present in the previous war, she is not affected by the four day time loop.

During Hollow ataraxia, Emiya Shirou finds that each time he runs into Caren, he dies. Eventually, Shirou ends up defending Caren from the dark creatures that have appeared since the time loop began. He is finally able to meet her without dying, but finds that he is experiencing strange feelings of anger and hatred and not acting like himself.

Shirō is eventually revealed to be an imposter or in the very least, not completely himself; he is actually Avenger in the guise of Emiya Shirou (the real one is either not present in the game, or being possessed by avenger). She continues to offer advice to him, perhaps knowing his true identity(s) all along. She and Avenger develop intimate feelings for each other.

It is also revealed that the woman who finds Bazett and nurses her back to health is Caren.

Trivia

 * TYPE-MOON revealed that Caren was originally planned to be a minor character in an earlier work, Tsukihime.
 * Caren's surname, Ortensia, means hydrangea.
 * Caren (可憐) and Kirei (綺麗) (homonym for Kirei, 綺礼) are synonyms in Japanese, giving an indication of their related roles in the Fate/stay night series.