Disclaimer: The image above is what the stone on Yui's choker looks like.
Yui woke up and took a shower. She ate her breakfast and went to work. Just like every day. She lived in an apartment in Ikebukuro. She came from a very wealthy family from Kyoto, yet she still left her family and moved to Ikebukuro of all places.
Yui seemingly never smiled. Of course, she did in her life, but they were rare and almost unnoticeable. Her father always attempted spoiled her and her sisters rotten. But unlike her sisters, who loved everything their father gave them, she didn't see the value in all the toys that were given to her. Yui felt smothered with no breathing space, especially when she knew they were spoiling her so they can simply marry her off later. She only felt like she could be herself when she was with her grandmother. It was funny, while her grandmother always had a sweet smile on her face, Yui always wore a blank face. Her grandmother would tell her things that none would imagine a child would be interested in, all with the same sweet smile. Yui's favorite topic had always been the topic of death: what happens to our bodies, what happens to the soul, what the final moments are like, etc. It was through these types of conversations that she learned how to read and understand people so easily. It was also during one of these conversations that her life changed forever.
Flashback: 10 years ago
"What's wrong Yui? You seem sad."
"I have a question: Dad gave a necklace with a unicorn on it. He thought I would like it. What made him think that?"
"I have a feeling it's because it's the fact you're a girl. Your father has always been rather old fashioned and rather stereotypical."
"Now that you mention it, he always tells me I shouldn't play with certain things or read certain books because they're boy's things."
"I'm taking that necklace he gave wasn't to your liking, was it?"
"No, it wasn't."
"Well then, you can have mine if you want."
"You mean the chocker your wearing now?"
"Yes. But, I'll admit it might be hard adapting to the new form that comes with it."
"New form?"
Then, her grandmother's skin turned into white scales, her teeth seemed sharper as if they were fangs, and her brown eyes turned blood red and looked like that of a snake's. Yui didn't even flinch, she still wore that blank look on her face.
"Are you sure I can have it?"
"Of course, besides you seem the best option out of my 3 grandchildren. You actually took the time to listen to me and genuinely enjoy our conversations. You identify the fact that your father spoils you so he can have leverage over your life when you grow up. Even though you never seem to show your emotions, you let others know how you feel no matter the consequences. You seem to have an instinct for deciding who to trust and can see through people's facades. Meanwhile, your sisters are becoming spoiled airheads. They might be sweet now, but when they're older they'll act like spoiled princesses who won't survive without daddy's money."
"You said it would be hard to get used to. Does that mean physical or emotional?"
"Emotional. For me at least, it was hard to touch my skin and feeling scales instead of soft warn human skin. To accept that my teeth now carried poison in them that could have any effect I wanted. That my eyes no longer looked human. That I wasn't human anymore. Though knowing you, Yui, I doubt you'll have as much trouble as I did. But, then again I could be wrong. Besides, you'll have Orochi to help if you need it."
"Who is Orochi?"
"That would be me."
Suddenly, a giant white snake with the same red eyes appeared behind her grandmother. It stared at Yui.
"He is a serpent spirit whose job is to serve those who own the necklace and do their bidding. Including keeping them alive for as long as possible."
"Is that why you're still alive despite your illness?"
"Yes, though he can't keep me alive forever, and when I die the choker will fall off."
"Isn't that what you wanted? To know what death is like?"
"Yes, in a way it's always been my dream. But, I couldn't leave Orochi without a suitable owner."
"So you chose me based on who you thought was most competent."
"True, but that doesn't mean I never loved you. You are my favorite out of not only my grandchildren but my entire family. I'm honestly sad how my son and his wife, your parents, turned out. They became so spoiled due to our family wealth that they lost sight of how the world works. It's like a disease and they spread it. I was happy to find out that I wasn't the only one immune to it. Because of our conversations, I've to love you as my own, something that stopped for my son."
Yui climbed on her grandmother's lap. Her grandmother cradled her in her rocking chair and began to stroke her hair.
"When you die, can you tell me what it's like?"
"I think it'll be rather difficult, but I'll try."
"If you are going to die, then can you sing me that song one last time please?"
"Which one?"
"That one. The one about the prisoners."
Her grandmother chuckled and began to sing the song wich others found odd and offsetting, but one that Yui loved.
Slowly, Yui fell asleep and when she woke up, she found herself in her grandmother's cold dead arms while the chocker was now on Yui's neck. Like her grandmother, her skin was now scaley, her teeth like fangs, and her brown eyes blood red looking like that of a snake's. Orochi then appeared in front of her.
"She died while you were asleep. Per her final orders, you are my new master. I shall now serve and protect you until the day you die. You can alternate your appearance at will and I shall follow wherever you go. If you need me simply call my name or give the order."
"I assume that when I don't need you you'll rest inside the choker and also that we are now somehow psychically connected based on your explanation. But you aren't in this because you want to, are you? I suppose you get some kind of payment?"
"Clever girl. I can see why you were chosen over your other 2 sisters. You are correct. I would like to receive human souls as nourishment. I admit that I can not die, though consuming human souls would be to my personal preference."
"But, to get human souls we need to kill others, right?"
"Yes, however since you are my master you have control whether I eat or not. So the murder is completely up to you."
"I see. By the way, I understand you being a giant serpent spirit. But, what abilities do you have?"
"I have enormous strength, can shrink into a smaller size, become invisible to all but you, extreme speed, venom in my fangs to which my master controls the effect to a certain degree, and can turn others into stone with my eyes. However, my stone gaze doesn't affect my master so don't worry."
"Can I use those powers?"
"To an extent, yes."
"About the venom, you said I control the effect. What does that mean?"
"It means you control what it does. If it kills, makes others ill, causes mental breakdowns, causes nightmares, inflicts some physical deformity, heal others, cause a certain emotion or reaction, etc. Pretty much any kind of effect of your choosing, you can even add certain conditions for the effects. However, there are limits to what it can do. For example, it can't make someone fly or read minds."
"I see."
"I apologize, master, but you should probably inform the others of your grandmother's passing."
"No, they'll discover her body when they come looking for me when it's time to go home. For now, let me sleep a bit longer. Wake me if anything happens."
Flashback: End
A few days later, her grandmother was buried in the family graveyard. In the next 8 years, Yui grew up living her life as if nothing happened. Except when she visited her grandmother's grave, she would continue talking to her as if she was still alive. As promised, Orochi was always there. He helped her adjust to the loss of her grandmother and her new abilities.
When she turned 20, she moved out despite her family's protests. She managed to get a decent apartment in Ikebukuro thanks to her inheritance from her grandmother, which she had gotten the largest piece of. Despite the inheritance being billions, her apartment was still simple and she still got a simple job to support her. Like her grandmother had predicted, her sisters became spoiled princesses who didn't care about anything besides their father's money. She was also right about them spoiling her to use it as leverage to control her life. Thankfully, Yui used the money she received to pay back her father for everything he bought her. So now she didn't worry about him anymore.
She was now 22. Living in Ikebukuro wasn't that hard despite all the criminals and gangs lurking around. Her grandmother frequently took her to places like this, these "field trips" let her see the world beyond what the rest of her family showed her. Yui also met many people in Ikebukuro who went from acquaintances to friends. There was Shizuo, Celty, Simon, Shinra, Anri, Kadota, Walker, Erika, and Saburo. She still didn't smile despite enjoying these new friends.
She never personally met Izaya Orihara. Though she would frequently see him at the supermarket she worked at or getting chased by Shizuo. Yui knew she should stay away from him, that seemed the smartest decision. However, whenever he came to Ikebukuro she was somehow aware of it. Based on what she had observed he seemed infatuated with humans and enjoys playing with them. But based on what she was told, he used that as a way of protecting himself from being hurt due to how fragile his heart is. It made sense in a way. But, she still had to make her own judgment of him.