Nu-13

Nu-13 is a fragile and thin girl with pale skin, large, sunken red eyes, and gray hair. Most of her hair is worn in a very long ponytail, the end of which is attached to a sword-shaped metal object. On her face is a seithr regulator, which she wears as part of an eye patch. The symbol for a Prime Field Device is tattooed between her collarbones.

When in standby mode, Nu's clothing is a simple light blue one-piece jumpsuit with short leggings and long sleeves which wrap around her middle fingers in a way similar to bridal gloves. There are metal harnesses on her jumpsuit on the nape of her neck and under her breasts. Over this, she wears a white and blue cloak with various rings attached to it. These rings are adorned by red talismans with various symbols on them. The cloak has a huge collar, held upright by a metal piece imprinted with Nu's Prime Field Device symbol. Nu's battle gear is very similar to that of a mecha. Her face is covered by a visor with horn-like antennae and a single, glowing red eye. Over her jumpsuit, she wears a harness which is attached to a blue jewel and to her shoulder guards. These shoulder guards are blue and white, with a gray part on the nape of her neck, and there are two seithr regulators on the back of each shoulder. The shoulder guards are attached to her arm guards, which have small blades similar to her swords mounted on them. Her battle gear's most distinctive feature is the plate armor covering her legs. These armored leggings are mounted with blades that serve as weapons for her. Nu's hips, upper arms, and armored leggings have glowing green lights on them.

Nu is able to telekinetically control eight swords, which she uses as weapons in addition to the blades on her legs and forearms. These swords often float behind her in a pattern similar to wings. While in her battle gear, there is a small black marking in the shape of a sword on Nu's jumpsuit.

To almost everyone, Nu is cold, mechanical, and apathetic. She is solely focused on her mission to fuse with Ragna, and will not allow anyone to come between her and that objective. When away from Ragna, Nu's speech pattern is robotic, using complicated and professional language, and vocally exclaiming when she is loading or commencing the extermination of a target. Like this, she will often analysis her opponents and assign them warnings, coldly assessing their strengths from a glance.

But to Ragna, everything changes. When around him, Nu is filled with emotion, lusting after him and breaking her mechanical language. She will vocally call out to him with sexual innuendo, taunt him about his darker feelings, and threaten to kill him, all while baring a smile filled with glee. It appears her feelings of "love" towards him are genuine, but love to Nu is simply fusing with Ragna to become the Black Beast - nothing more or less. She won't allow anyone else to touch him, she only wants the Grim Reaper to herself, and as far as she is concerned, everyone else is an obstacle in the way of her happiness.

Eventually, Nu's mechanical expressions faded away, and she is nearly always filled with intense emotions, particularly on killing others and seeking Ragna. She went insane from not being with him, and would do quite literally anything to become one with him. She would even defy the orders of those who command her, being derogatory and showing a sharp tongue to anyone who isn't her love. Nu has no hobbies, bar thinking about Ragna. Nu is part of the yandere trope of characters that appear in Japanese media.

As a Boundary Interface Prime Field Device, Nu is completely immune to the effects of seithr and can freely roam the Boundary with no consequences. Since she is one of her species that is modelled after Saya, she has incredible Ars Magus aptitude, merited by her ability to use the Lux Sanctus: Murakumo. When activated, Nu has its bladed armor and floating eight blades at her disposal, and she excels in masterfully using them against her opponents.

An ability that Nu excels in using is deploying her Petals, floating black spaces that deploy a myriad of blades; in gameplay, this is her Drive, Sword Summoner. There are no limits to the black blades she can produce with this ability, some of them ranging from typical swords to blades that run along the ground, catching at her opponent's feet. Even without this ability, Nu is blindingly fast and strong, capable of various acrobatics and blows strong enough to knock opponents meters into the air.

Nu has a Life Link with Ragna - this means that neither can die unless they both die in a similar time frame; because of this, both are unconventionally immortal. Even without her life link, Nu is extraordinarily difficult to kill, being able to regenerate her damaged body at inhuman speeds. Even when she is damaged or in pain, Nu's will to fight through to kill her opponents means that she is unstoppable. Even the Arcus Diabolus: Bolverk has not enough stopping power in its charges to stop Nu from hunting her enemies.

As this prime field's mentality has begun to unwind, she has become more vicious and willing to destroy. She is far easier to annoy and her quickness to anger in her pursuit of Ragna is a weakness that is easy for strong opponents to exploit. After absorbing Take-Mikazuchi's core, Nu is a threat that only the strongest can eliminate - she can fight evenly against Jubei and Hakumen, two of the Six Heroes, but her deteriorating mental state makes her an easy target for individuals such as Yuuki Terumi.