Yui

Yui (結, Yui) is Akira's older twin brother. He is the former user of the nil calculus.

Apperance
Unlike his twin brother Akira who has a feminine-like appearance, Yui has a more masculine appearance. He has dark blue hair that reaches just above his shoulders and blue eyes. He has thick eyebrows which accents his intimidating aura. He wears a green fur hooded jacket with grey fur on the hood, dark blue pants and dark brown shoes with darker brown soles and toes.

During his childhood, he wears a simple white kimono and footwear which seems to be made of straw. After calling the power of the nil calculus, it was attached to his chest ever since.

Personality
At his first appearance, Yui appears as a cold, emotionless individual who also shows signs of apathy and an antisocial personality, shown when he mercilessly freezes a group of thugs to death after he has asked them where his twin brother was. Upon reuniting with his twin brother, he displays a more robotic and soulless demeanor, following Akira's every whim such as constructing an elaborate ice palace or bringing things he asked for, but on one condition where Akira must never leave. This made him protective, but to an unhealthy and possessive degree.

It's later revealed that this empty and nihilistic personality with the exception of his only grounding center, Akira, is caused by the effects of Nullstone, which in exchange for strengthening his already strong cryokinetic powers, leaves him nothing but an empty shell of what he once was, only possessing remnants of the core things that made him attach the Nullstone in the first place, which in this case is his younger brother, Akira. The effects of the Nullstone rendered him hostile, indiscriminately attacking anyone who even dares attempt to take his brother away from him, and judging how he felt no remorse upon annihilating his entire village and holding his twin captive in the ice palace he constructed himself, also nullified his sense of morality.

Back when he was a child, he was the more mature, intelligent, and responsible of the twins, loving his twin brother dearly and is protective of him. He along with Akira desired to see the outside world but is prohibited to do so due to their "cultural" duties as Yuki-onoko. He was once brought to see the other Yuki-onoko in the village, and seeing his deplorable living conditions and unstable mentality, made him scared of what would happen to his twin brother. This led to him volunteering to become the chieftain of the village to spare Akira from such a fate. The duties as a chief and the emotions he can sense from the women who feel inferior slowly corrupted him, swallowing him in hate and animosity by time and again. However, his bond with Akira pulled him from such thoughts, thinking of him as his only salvation. As his mental health dwindled, he started having obsessions with what was pure and tainted, considering Akira to be pure of heart and wishing to protect him.

Upon hearing that Akira wet the bed, and in suspicions that he may be able to start serving as chief. Yui grew distressed and worried about the implications and resolved to never let the women of the village find out about the changes in his brother. In desperation, he assembled all of them and declared that he will appease them to their wishes in exchange for letting Akira leave the village, or else he will kill himself. But, upon overhearing a group of women violating his orders by sending someone to kill Akira, Yui's delicate mentality snapped, and while on the run from the women attempting to cage him, he laments and curses his village being a cycle of death and rebirth, mentally declaring that the village is dead. He then took the Nullstone in despair and attached it to himself ever since.

After having the Nullstone removed from him, Yui is shown to be more relaxed, solemn, and honest. He addresses Kabane, who stopped him from rampaging any further with the honorific -dono, which indicates a high level of respect for the Ghoul. He readily shares his knowledge of the Stones and the Kemono Incidents upon questioned, and suggested Kabane keep the Nullstone, saying its the least he can do. He laments how he deserves to be alone, after everything he's done, including how he destroyed the snow village and all the women and innocent children in it, calling it an unforgivable sin. After being reminded by Akira, that they're twins and no matter the burden, they can withstand it if they're together, light seems to be restored to Yui's eyes.

Background
In the snow village of mount Iwanki, Him and his twin brother, Akira, were born. It has been said that a male Yuki-onna in their village is born once every hundred years. When growing up, the two would go near the barrier that protects the village but, kept getting caught. In a magazine Akira found on the mountain, he and his brother found about the Inugami detective agency and Tokyo.

When the women of the village are deciding which of the twins should be chief, Yui volunteered to be it as he didn't want Akira to suffer. As chief, he would have sex with the women to make sure the village survived. One day, his brother thought he had wet the bed and burned it as he didn't want the others to find out. This worried Yui as his brother didn't wet the bed but, has started ejaculating.

He asked the women of the village to let out of the barrier and promised them he would impregnate all of them if they accepted his request. They agreed and Yui talked to his brother as they are going near to the barrier. He told him that he was leaving the village too but, as they are getting close he left him. Before he was completely out of Akira's line of sight, he told him to go to Tokyo, find the kemono office, and they will see each other again.

Later, Yui would go to the place where it posses the Nullstone that created the barrier for the snow village to escape the women. He had enough and put the Calculus in him, wiping the village in the process. As the calculus possessed him, he only remembers what was important to him, his brother. Leaving in search of him and he was later reunited with him.

Powers & Abilities
Because Yui is a Yuki-onoko he possesses the powers the species are known for.
 * Cryokinesis: Yui has the power to control and create ice. His cryokinetic abilities are noted to be much more stronger and precise than his twin brother, Akira, greatly alluding to either his natural talent or due to the effects of the Nullstone he was attached to. His craftsmanship is immensely detailed and stunning, where he managed to create an elaborate ice palace for Akira to live in, and create subsequent complex types of furniture such as a bed and a bathtub. His abilities in combat outclass Nobimaru in terms of raw power alone, able to spawn bursts of ice in his vicinity to ward off any attacks and counter-attack in the shape of huge, ice spikes that penetrate his enemies, killing numerous clones of the Kitsune in the process. He is also able to create more elaborate combat gear, such as an innately detailed kabuto helmet and a pair of vambraces and gauntlets, as well as using a pair of block-shaped ice to aid him in combat both defensively and offensively. His main method in fighting is to freeze his enemies into living ice statues, killing them in time due to hypothermia if they aren't sufficiently thawed. Nonetheless, his powers may be formidable at the beginning, but he also has a time limit, shown when he collapsed on his knees and remained motionless, due to the effects of the Nullstone having caught up onto him despite his resistance to cold and slowly sapping his life away.

Akira
He is Yui's younger twin brother. Since they were young, Yui loves his brother and cares for him greatly, protecting him from harm even from their own clan. As his mental health started slipping down, he clung desperately to the peace and stability that his bond with Akira provides, even to the point of unintentionally giving his younger brother an inferiority complex by saying "Never do anything but smile". When he learned that he is deceived and the women discussed how Akira is most likely dead, he snapped and destroyed the village in retaliation. Even when swallowed by the overwhelming power of the Nullstone, Yui held onto the things important to him, which is his younger twin brother. Upon getting a wake-up call from Akira and being reminded of the promises they made to each other, his bond is rekindled with him once more, to the point where Akira follows him in the first half of his self-reflection journey.

Etymology

 * Yui's name contains the kanji for "conclusion" (結, yui).

Trivia

 * His favorite foods are sweet things especially sweets topped with ice cream.
 * He has nothing in particular when it comes to his least favorite food.
 * The first thing he did when he came to Tokyo is to go on a sweets run.