(Clarification anya is an adult now)
Y/N just Graduated and landed a office job in an Government funded company. Y/N made a lot of friends there and he developed a Crush on an Pink Haired Girl.
Her name was Anya Forger she sometimes invite him for lunch trips in cafe's. They always talk about their life but when he asked her. About her childhood she Just avoids it so he didn't want to pry anymore.
But what Y/N didn't know is that Anya was actually a spy and a assassin. Not only that but she was a telepath too. And Anya only Took this job for Cover and because Y/N works here. Unknown to Y/N though that They already met in Highschool they were classmates there. And Anya Developed an Obsession over you. And Anya already Knows that Y/N has a Crush on her.
One day Y/N invited Anya to a restaurant. Anya came and then they ordered their food. But after they ate Y/N finally confessed.
(Y/N): Anya I've been meaning to ask you this for a long time now. Will you please be my girlfriend?
Anya: of course Y/N i feel the same way to you.
After that they moved in together but throughout the days Y/N spent with Anya. He always asked why Anya was always going out at night. But Anya just avoided the Questions.
(Y/N): Hey babe dinners ready!
Anya: I'm sorry babe but i have to go out tonight.
(Y/N): What again ok just be safe.
Anya: Ok honey I'll join you in the bed later.
Y/N ate diner and prepared going to sleep. Y/N was a heavy sleeper that why Anya can wash of the blood in her clothes. When she goes for some missions. But One day Y/N woke up and saw the bathroom lights open. He looked inside to see Anya washing off the blood in her clothes.
Y/N was shocked and stumbled backward making a loud crash sound. This was heard by Anya who left the bathroom. She saw a terrified Y/N backing away from her.
Anya: Y/N baby what are you doing.
(Y/N): W-why a-are there bloodstains in you c-clothes?
Anya: Baby this is nothing just had a nosebleed.
(Y/N): Your lying I'm going out.
Anya: I'm sorry babe but you can't leave me.
(Y/N) But you kill people your a murderer.
Y/N Quickly packed his bags and was about to go out but was stop by Anya.
Anya: Where do you think your going babe.
(Y/N): I'm leaving because I can't be with a killer!
Anya: You're not going anymore
Anya chopped Y/N's neck knocking him out she then drags Y/N to another room. As Y/N wakes up he saw he was in an unfamiliar room. The door opened and Anya Went inside to check on him.
Anya: Ohh Honey your awake.
(Y/N): Anya! Why are you doing this. Are gonna kill me.
Anya: What? Honey no i just don't want you to leave me.
(Y/N): W-why are you killing peoples
Anya: because I'm an assassin babe.
(Y/N): Please just let me go Anya.
Anya: Let You Go! Never your gonna stay with me forever look we already have a kid on the way.
Anya showed Y/N a pregnancy test and Y/N was shocked by this. He just can't leave his child.
(Y/N): Ok ok I'll stay with you just let me go please.
Anya removed Y/N's restraints as she removed the rope. Y/N suddenly pushed Anya away and run for his dear life. He was on the streets running from Anya then he stopped to get some air. When a hand crept through his shoulder he looked back to see a couple. A blonde man and A Black-haired woman.
???: Hello there sir I'm loid and this is Yor. Can i ask you if your Ok.
(Y/N): I was being chased by my girlfriend, she locked me in a room thank god i escaped.
Loid: Oh that's terrible if you want you can come with us. We were visiting our daughter. We'll protect you ok
(Y/N): Thank Mister.
You walked with the couples and to an apartment. You found it weird because that's where your Girlfriend lives too.
(Y/N)'s Mind: their daughter and Anya lives in the same apartment i hope i don't run into her.
But then suddenly Y/N felt a familiar person behind him. He slowly looked behind him to see Anya standing there.
(Y/N): Help! Mr. Loid it's her!
The couple turned around and they saw Anya there.
Yor: Anya! What are you doing to this poor man.
Anya: Mother? Uhmm this is my Boyfriend hehe.
Loid: Anya! This man told me that you locked him in a room.
Anya: But Father he was gonna break up with me.
(Y/N): Because I know you killed someone your an assassin.
Yor: Come on Anya. When i was an assassin nobody found out i was an one.
Loid: Hey kid. What's your name?
(Y/N): my name is Y/N sir
Loid: Y/N I'm truly sorry for the actions of my daughter. But please this is her first relationship so do work things out.
(Y/N): I'll try sir. And congratulations for having a grandchild.
Loid & Yor: G-Gr-grand Child!!
Loid: You knocked up my daughter!
(Y/N)'s Mind: Well I'm dead.
The end
Not much of a yandere tho.