A cheerful girl with short brown hair, visible pink blushes on her cheeks, large round eyes and soft pads on fingertips named Uraraka Ochako was on her way to U.A. High School. The high school which she was currently attending to learn how to be a pro-hero had recently built some dorms for students to live in. Considering recent events, including the villain attack during summer camp and All Might's retirement after fighting a certain terrifying villain, she understood why the school deemed it necessary to build the dorms for the students' safety.
Speaking of safety, Ochako frowned in worry as she thought about her friend, Midoriya Izuku. A boy with bright-green eyes, fluffy dark-green hair and freckles on his face, the boy had suffered much due to his fight with an infamous villain known as Muscular during the summer camp attack. Her heart could not help but clench at the awful memory of his broken body as he cried in despair due to failing to save Bakugou Katsuki from getting kidnapped by the villains. Although Izuku's injuries had healed and Katsuki had been rescued, she still worried for the boy whom she could not help but… like a lot.
She was so distracted by her thoughts that she failed to notice that a small blob of pink slime slipping into her luggage bag…
-At the U.A. Dorms…-
After her home room teacher, Aizawa Shouta, told her and her classmates their room arrangements, Ochako immediately went to her room to unpack her belongings. Due to being poor, she did not have much to unpack. However, just as she was opening her luggage bag, the blob of amorphous pink slime which had been hiding inside it suddenly leapt out and covered her face. Too shocked by the sudden attack to even scream, she struggled futilely as the slime started to engulf her face and even probe into her ears. After a few seconds, her weakening struggles stopped entirely as the slime oozed into her head through her ears.
As soon as the last of the slime entered her head, her eyes opened and she wore an uncharacteristic smirk as she thought, "Finally, I thought I'd never get myself a new host!"
Getting up onto her feet and looking over herself, the obviously parasitic being inside Ochako thought to itself, "Hmmm… not a bad body to stay in if I have to say so myself." It then looked into Ochako's memories and thought, "And a student of U.A. High School itself no less. I can definitely make use of this situation!"
"W-what's going on? Why am I not in control of my own body and why do I have a voice in my head?!" cried the real Ochako who was basically trapped in her own mind.
The parasitic being made Ochako's body strike a villainous pose as it mentally replied to her, "Well, if you must know, I'm what you may call a villainess. You can call me… Brain-Jacker."
Realisation hit Ochako as she replied, "You're that slime that jumped out of my bag and onto my face just now!"
"Bingo! I barely survived the Kamino Incident which forced All Might to retire after defeating that monster of a villain," replied Brain-Jacker who mentally scowled as she added, "If it wasn't for the fact that I could move around and survive for a few days without a host, I would have been trapped under all that rubble along with my last host."
"Y-you won't get away with this! Deku will notice something is wrong with me and…!"
"No, he won't," said Brain-Jacker who then smugly explained, "You see, my quirk, [Parasite Slime, allows me to gain control over any biological woman that I inhabit. Not only that, I can read your memories to not only mimic your behaviour and mannerisms perfectly but even uncover certain secrets about you such as your crush on a certain someone."
Ochako was panicking as she mentally squeaked, "Y-y-you know that already?!"
Brain-Jacker smirked and licked her finger with eager anticipation as she mentally replied, "That's right, and I know exactly what I'm going to do while I'm in your body. You see, I'm the kind of villainess who enjoys toying with my victims by making them have sex with men." Suddenly, she saw images of Ochako's fears and her visage became a disgusted scowl as she added, "Hey, just because I enjoy making my victims have sex with men doesn't mean that I don't have standards!"
"So… you're not going to force me to have sex with random men or break Deku's heart?" asked Ochako who was hoping that such would be the case.
"Don't get me wrong, I am certainly in control of your body. However, if your emotions get too strong for me to… ignore for the lack of a better term, I'll start feeling them as though they are my own. Needless to say, I'd rather not feel your feelings of despair and hatred if I can help it," Explained Brain-Jacker who then added, "Of course, there is the option of suppressing your consciousness so that I do not have to put up with your emotions and thoughts but even I can't forcefully suppress it indefinitely unless your let me do so. The longer I forcefully keep your consciousness suppressed, the harder it will be for me to do so without losing control of your body. That said, I can freely alter your libido and fill your mind with lewd ideas so there's always the possibility of you becoming the cum-dump of your class by the time I'm through with you."
"But you won't as long as I strongly refuse to submit and you wish to avoid feeling my negative emotions," said Ochako who was beginning to have a grasp of Brain-Jacker's abilities and limitations.
Brain Jack nodded and replied, "That is undeniably true, unless you change your mind at some point and decide to be a complete slut."
"So what will stop you from making me so… pent up that I would allow even Mineta to have sex with me?" Ochako was talking about Mineta Minoru, a short boy with purple balls in place of hair on his head and a complete pervert whom the girls had beaten up on more than one occasion due to his perverted antics. Understandably, Ochako did not want to even think about the idea of having sex with him.
"To be perfectly honest, it's mostly up to your feelings and morality. If your feelings for that Deku-boy wins out, the worst I can do is make you his personal cock sleeve," replied Brain-Jacker.
"T-that's already more than awful enough!" cried Ochako.
"And I'm a villain so I don't really care as long as I can have my fun," replied Brain-Jacker who then gloated, "Tonight's going to be a fun night!"