Nue

The is a yōkai that was created by Root. It is the result of Project Gozu Tennō, designed to be the ultimate weapon harvested from Hashirama Senju's cells. While spearheaded by Tanuki Shigaraki, it was passed on to his daughter Sumire Shigaraki with the intention to wreak havoc in Konohagakure.

Background
As Danzō continued his amoral methods to protect Konohagakure, realising the potential power within the First Hokage's very cells, he harvested some of them. Placing Tanuki Shigaraki, one of his top researchers and most loyal followers on the project, the goal was to create an ultimate weapon. After Danzō's demise, the blindly loyal Tanuki grew to resent Konohagakure for "betraying" Danzō. Desiring to avenge him, he relentlessly continued his research into finishing the project, all while training his daughter to properly wield the power of the beast in the event he died before its completion. Eventually, he met his own demise after passing the Nue on into Sumire. In the following years as Sumire established a new identity, she began nurturing the creature's development, forming a strong bond with its summoner.

Personality
While seemingly a rampaging beast drawn to people of negative emotions, the Nue is in fact extremely loyal to its summoner, Sumire. Despite the dark nature of its creation, it is filled with love towards Sumire, having come to view her as its parent after spending years being nurtured by her. It was even willing to sacrifice itself to ensure her survival.

Appearance
Normally, it appears as a shapeless dark purple mass. While absorbing enough chakra, Nue takes a tangible form that is reminiscent of a chimera: it is large and has a monkey's head with large fangs and a white mane, a tanuki's body, a tiger's legs, a white snake for a tail and glowing yellow eyes.

After the Gozu Tennō was destroyed, the strong personal connection Nue forged with Sumire allowed its core to be sealed within Sumire's left palm, taking the form a small red dot that acts as its eye.

Abilities
The spectre is able to sense the negative emotions in an individual, using that to select its target. Once it possesses someone, it preys on their emotions, making them give in to their darkest impulse leaving the victims themselves unaware. Only when the victim overcomes their negative emotions or is knocked out will the spectre be forced out from its vessel. It also appears able to feed off their victim's chakra as sustenance, using it to take a physical form to attack independently.

In its fully tangible state, Nue is shown a unique form of regeneration, able to grow several smaller versions of the limb from the stump. It is very strong, able to smash through multiple giant stone walls and highly resilient as launched kunai merely bounce off it. Its endgame ability is to use the collected chakra to self-destruct in a powerful explosion. It is also able to use Space–Time Ninjutsu to teleport, escaping giant barriers.

Academy Arc
On the night before the Academy entrance ceremony, when Denki Kaminarimon was struggling with his strained relationship with his father and his constant bullying, the spectre possessed Denki. The following day, the possessed Denki lured his bullies onto a defective train, having set up another train to collide with it. Boruto Uzumaki, having saw the dark chakra emanating from Denki went to help. Making him realise that he can't harm other people to solve his problem, Denki snapped out of his possession, expelling the spectre from his body. Weeks later, when Metal Lee was growing ashamed at his social-anxiety getting in the way of his performance, the spectre possessed him.

The following day, the possessed Metal attacked Boruto, Shikadai Nara, and Inojin Yamanaka as the ones who called him on his cowardice. With his possessed aggression, Metal quickly forced the other students to flee. Ultimately, Shikadai was able to subdue Metal with Shadow Imitation Technique. As Metal began to break free, he tripped and landed on his head, knocking himself out and rejecting the spectre. Later, when Boruto attempted to perform a Summoning Technique despite no blood contract, the spectre used the chakra to take a solid form. As it attacked the students, Konohamaru Sarutobi stopped it with his Rasengan, ultimately destroying its form before dispersing as a mass of purple light.

Later, the spectre possessed an expelled student from the Academy, latching itself to the boy's bitterness at failing to be a ninja. The possessed boy began causing mayhem in the village with his Fire Release. He was ultimately approached by Boruto again, along with Shikadai and Iwabee Yuino. The boy was forced to retreat, only to be defeated by Mitsuki, again forcing the spectre out. The following day, the spectre possessed a ninja working on repairs on the Academy, latching onto his frustration at the students making his work more difficult. As the ninja began causing more destruction to the Academy, the students worked together to stop him, ultimately subdued by Boruto and Mitsuki. Later that day, Shino Aburame, the class' teacher, was very distraught at both his inability to command respect from his students and feeling responsible for the further damage to the Academy. The spectre then possessed Shino, making him decide that the best way to deal with problem students is to get rid of them. The possessed Shino then sent a letter to Boruto, Mitsuki, and Shikadai to meet at a training field for extracurricular training. Once they all met up, Shino made his dark intentions known. Shino then encased the entire area with his insects, forcing the students to flee. After a prolonged chase, Shikadai devised a plan to stop the rampaging sensei. While Shikadai and Boruto distracted Shino and lured him into the water, Mitsuki ensnared Shino and electrocuted him, expelling the spectre.

Later, a shy boy from another class, Magire Kakuremino, began stalking Sumire Kakei. When he finally worked up enough courage to ask her out, the similarly shy Sumire turned down the offer, leaving Magire in despair. This eventually drew the spectre to him. Once possessed, Magire's stalking turned much darker, eventually leading to a kidnapping. As Sumire's classmates went to her aid, the possessed Magire began attacking them, overwhelming them with his concealing technique. Ultimately, Chōchō Akimichi was able to reason with him, insisting that desperate antics would appeal to no one and must work to improve himself if he wants to be acknowledged by others. Accepting his words, Magire expelled the spectre and returned to normal. However, the prolonged possession allowed the spectre to steal much of Magire's chakra, causing the student to collapse. In the following days, the spectre's rampage began escalating from simply within the Academy to throughout the village. Later, another attack caused damage to the Konohagakure Water Purification Plant, which also seemingly-injured Sumire Kakei.

As Sumire realised that Boruto could see the spectre, she began watching Boruto's movements, strategically unleashing the spectre in areas away from Boruto so it could attack with enough time to gather chakra. As Boruto and his friends realised they were being watched, they decided to get help from their classmates to better patrol the village. Once leaving the post office, a masked Sumire decided to continue her work on postal chief Komame. Once having possessed Komame, Boruto and his friends revealed themselves, having anticipated this action. While Sumire retreated, Boruto and half his allies fought the possessed Komame. As one of Komame's workers Tayori began to get through to the postal chief, it allowed Boruto and Iwabee to knock out Komame. Boruto tried to subdue the spectre with an explosive tag kunai, but it quickly disappeared.

Later, realising that she was found out, Sumire left the hospital to begin the endgame of her plan. She was intercepted by Sai Yamanaka's unit, who initially managed to subdue her. She managed to escape by unleashing the Nue, who's tangible form began to unleash a rampage. Kakashi Hatake made the first strike, severing the creature's tail, only for it to grow dozens of smaller tails in its place to steal nearby people's chakra. Kakashi than organised the shinobi to form a barricade to keep the Nue contained. Meanwhile, Boruto, who was confronting Sumire, began to have a strange reaction to the events, causing his right eye to act erratically. This drew the attention of the Nue, which teleported above Boruto. Wanting to complete the Nue's development, Sumire offered herself to the creature, which swallowed her whole before disappearing into a void.

As Boruto and Mitsuki followed through the void into the pocket dimension, they quickly resumed battling Nue. While they were able to restrain it, Sumire quickly came to its defence. Boruto continued to reach out to her, insisting that violence was not what she wanted unlike her father. Desperately clinging to her father's request, she ordered Nue to take her chakra and finish the plan. Nue instead ignored Sumire's order and instead snuggled against her. While Sumire was confused by this event, Boruto deduced that having been nurtured by Sumire all these years, it came to view her as its parent. Insisting that Sumire doesn't want it to hurt people any more than she does herself, Sumire finally let go of her anger. This caused the Gozu Tennō to be destroyed and the dimension to begin crumbling around them. Nue then aided its surrogate parent and her classmates into escaping the dimension, causing it to be left behind from falling debris. Despite this, the strong connection it developed with Sumire allowed her to retain a piece of its essence, letting Nue live on sealed in Sumire's left palm.

Trivia

 * A Nue is a mythological Japanese creature said to have the face of a monkey, the legs of a tiger, the body of a tanuki and the front half of a snake for a tail.