Nyon ⁠˖3⁠˖

He can sleep forever if he is not disturbed.
He smokes marijuana.
The official YouTube video for "THEUSA" hints that she's jealous of Nyen. (Or, in theory, she tried to save him from Luther, but seeing the fear in his eyes, she chose silence as her own punishment and shame.)
He is masochistic, born from the absence of feelings and the inability to express them.
Nyon's hat would save him if he ever fell into the ocean [ 1 ]
He's quite smart when it comes to technology.

Luther Von Ivory
Nyon is Luther's first cat, and they get along well. "Although she sometimes prefers to stay out of trouble with Luther, she knows her owner's behavior is far more questionable and that she can fear him just as much."

Nyen
Nyen is Luther's other cat, besides Nyon. Nyen and Nyon seem to have a rivalry, and Nyon is easily intimidated by Nyen. But they are also quite peaceful and sometimes smoke marijuana together.

Randal Marfil
Nyon displays a certain fear of Randal, which is notably evident in Nyon's introduction, where he states in the fourth comic that bandanas make the world a little less frightening by covering Randal with them. Despite this, he seems to indulge Randal's every request, which could be due to his being Luther's younger brother, as well as his generally obedient nature. (Or perhaps he prefers Randal, seeing him as his true master, since he was so different from Luther.) Nyon is one of the few members of Luther's household with low danger potential (along with Sebastian). He is shy and tends to avoid other creatures rather than confront them directly due to his soft voice and withdrawn demeanor. Sometimes, if he feels like it, he relaxes and smokes marijuana with people (if they are okay with it).

He is a man of few words who speaks Russian. He can also be helpful and guide people to escape.

In Lucid 20, he tries to help the Rat Man escape, but ends up being devoured by Nana in Lucid 22. Like Nyen, Nyon shares most of his physical attributes with Luther. Nyon has shoulder-length light blue hair, styled in a pageboy cut similar to Luther's. He wears three false sideburns on each cheek and three false eyelashes on each lower eyelid. His eyes are surrounded by red circles. He always wears a black headband with two round, black ears on either side. His mouth is smaller than Nyen's. He wears a long-sleeved yellow shirt with the phrase "OK 1/2" on a white oval design on the side. He also wears light blue jeans and black pointed shoes. He is always seen slouching.

In one of the first lucid visions, he is depicted as a gray kitten. And in the 30th lucid vision, both his whiskers and eyelashes disappear.

Personality
Nyen is usually calm and serene, though he has a high opinion of himself, emphasizing how different he is from humans (partly due to traits he inherited from Luther). He isn't very talkative and tends to be aggressive and sarcastic with everyone. Nyen is the mainstay of the household and becomes territorial if anyone crosses him. He tends to be controlling and wants people to obey his orders, tolerating no disobedience that disturbs him. But with Master Luther, he obeys and will do almost anything Luther asks, even harm others. Nyen takes a particular, sadistic pleasure in killing his victims.

Nyen considers himself an "idiot" and enjoys bullying Nyon and Sebastian with his cruel pranks. Despite his malicious jokes, he enjoys reading romance manga, but only after Randal gave him one. Nyen has feline tendencies and is often mischievous.

Although in reality he is more aware, just like Sebastian, because he tries to avoid risks, and especially his Master who tries to go with the flow (probably he doesn't want to become an entity, or end up like the others).

Appearance
Like Nyon, Nyen shares most of his physical attributes with Luther. Nyen is a cat-man with shoulder-length pinkish-white (sometimes completely pink) hair, styled in a pageboy cut like Luther's. His eyes are always large and surrounded by red. He has two prominent eyelashes under each eye and two black whiskers on each cheek, which were later drawn with a marker in the second volume, and he has a clean-shaven face. His mouth is wider than Nyon's. His style is more alternative, though somewhat vague, compared to the other members of the family. He wears a black hat with two points, mimicking cat ears, and a black hoodie with the word "NEVADA" (or sometimes "NIRVANA") on the front. He also wears beige pants and black shoes.

He has long, dark nails, and is often seen holding a cigarette.

When he hunts, he wears a red mask with pointed teeth.

Relations
Luther Von Ivory
Nyen has a very positive relationship with Luther and does everything he asks. He is very devoted and loyal, and does everything he can to make Luther happy. Nyen is very attached to Luther and enjoys getting his attention.

Nyon
It's as if Luther mentioned that Nyen and Nyon are described as "brothers." Nyen sees Nyon as weak and bullies him when he's in a bad mood. He told Nyon to stay inside the box and proceeded to stab him.

These two have a slight rivalry, although they can sometimes be quite peaceful and occasionally smoke marijuana together.

Sebastian
Not much is known about their relationship, except that she is willing to be violent with Sebastian when he doesn't flush. Nyen commanded him as the highest-ranking pet in the Ivory House. When Sebastian was imprisoned in a body armor and forced to curl up like an armadillo by Luther's remote control, Nyen decided to kick Sebastian for her cruel amusement before leaving. Nyen also keeps watch when Sebastian tries to escape. Sebastian finds Nyen intimidating, so he tries to avoid provoking her as much as possible.

Randal Marfil
Judging by their few interactions, Nyen finds Randal annoying at times, especially when Randal acts like a troublemaker and ruins his plans. Nyen has even tried to strangle or physically harm him simply out of spite. He also gets angry with Randal when he mocks him by juggling his eyes and making fun of him for losing an eye because of a rat and a pineapple.
Kitten rug.
Nyen, a wonderful person with cats, noticed the “cat rug” outside the window and reacted by hissing territorially.

Trivia
He is considered sadistic, but also somewhat self-aware.
He presents himself as a Nirvana fan.
She's also a fan of Diana Ross, and although this may seem unexpected, it's quite common for rock and/or metal music fans to have a "less heavy" artist they particularly like.
Her "NEVADA" sweater was a gift from Luther.
His favorite alcoholic beverage is Red Dog beer.
Nyen's hat can be removed or moved, and sometimes he keeps things inside, like snacks.
Nyen is the Jack level of the Ivory House.
In Lucid 14, Nyen is described as the "bad bitch".
His real name is probably something from the Bible like Michael or Andrew or Jacob.
Because of his annoyance at no one pulling the lever, it could be that he has bacillophobia (fear of germs and dirt).
Unlike the others, he's similar to Sebastian: he doesn't have deformities or powerful abilities like Luther, Nana, or Nyon. The only exception is when he loses an eye during the Rat Men arc and in Lucid 2. In the end, Randal helps him by giving him an eye, suggesting that he might also be human (someone capable of feeling pain).