Tulpa

Tulpa(ツルパ; Tsurupa) renamed Sakura Nagamura(長村桜; Nagamura Sakura) is a strange phenomenon taking the shape of a young girl in DxD: Haremonogatari continuity. Being a Tulpa, she calls herself the amalgamation of all tennage dellusions and fantasies in Kuoh academy, being only visible to Ryuusei after he fantasizes the love triangle between himself, Lunamaria and Kotone.

Ever since her defeat, she became Ryuusei's shadow, saying that his unique mentality have an exquisite taste.

Appearance
Sakura takes the shape of a well-endowed, fair-skinned young girl with bright blue eyes, shining golden hair and soft face structures, a derranged scar across her neck as a sign that she's not completely human and a furious idiotic cowlick sticking from the top of her head. Being made from the collective thoughts of all students from Kuoh, she usually wears Kuoh's female uniform of white lined shirt, magenta skirt and black corset-cape.

However, despite showing such appearance to Ryuusei at first, Sakura claims that she has no real "true" identity, as she changes it accordingly to the collective thoughts of Kuoh's students, claiming that she used to be a handsome young man.

Being a Tulpa, she has no reflex or shadow, and lacks a presence altogether

Personality
Sakura has a vague, passive and quiet behavior, Ryuusei comparing her to a ghost as she stalked him around Kuoh without saying anything. Being a Tulpa, she claims she has no real personality, and can pretty much inpersonate any kind of archertype.

Despite this, Sakura seems to be able to be greedy, as she took residence inside Ryuusei's shadow so she could feed on his rather Chuuni dellusions.

History
Sakura has been a supernatural phenomenon ever since the foundation of Kuoh academy. Living and being shaped accordingly to the wills and thoughts of the students and school staff, she would first take the shape of a handsome man which would walk around and enchant the females around. Once the school became an co-ed, however, due to the massive amount of male students with dreams of harem and romance dellusions, she end up taking a more feminine appearance, similar to a stereotypical image of a foreign girl, and appearing only for those who succumbed to those dellusions.

She appeared for Ryuusei once he begins to imagine the romance between himself, Lunamaria and Kotone.

Powers & Abilities
Immortality - Being a concept rather than a true living being, Sakura is, in essence, immortal, since she can't be physically harmed and can exist as long as thoughts and dellusions about her concept exists, making her practically immortal.

intangibility - Since she's a concept rather than a living being, Sakura is also unable to be harmed or even touched without her permission, as any object will pass through her like she wasn't even there, and only slightly warp her image for a while. However, she can't harm or even touch anyone without consent either.

Telepathy - Being a tulpa, Sakura can read the minds of anyone within the grounds of Kuoh academy unless they use a form of mental protection for such. She can also communicate through mental link between herself and her target.

Master Illusionist - Sakura has the power over illusions, which are extremely powerful and almost tangible by the eyes of her victim. Being able to enchant her victims right through their minds instead of just visual contact, her illusions, while can't harm the victims, are able to deteriorate one's sanity until there is nothing left in their mentality, driving them into madness.

Shadow projection - Sakura is able to merge with one's shadow as a form of disguise and undectition.

Trivia

 * The images and appearance used are from the character Marie from the Dies Irae series.
 * Apparently, every people hears a difference voice from her, as Ryuusei describes her voice as delicate and soft, while Lunamaria describes it as hoarse and guttural.
 * Her human name, chosen by Ryuusei, Nagamura Sakura(長村桜) is a play on words on the japanese word for "mirage"(蜃気楼; Shinkirou).
 * Apparently, her big breasts came from the thoughts of the main character of the series Issei Hyoudou.
 * She has no favorite word.