Disclaimer: I do not own Boku wa Tomodachi ga Sukunai, Haganai, I Don't Have Many Friends, or whatever-the-hell-else you might call it. I'm just writing this because I had the idea.
Kodaka Hasegawa had just gotten home from a trip to the store, holding a bag containing small packages of pork ramen for dinner. He'd promised Kobato he'd make her pork ramen at least once a week to make her feel better about him always hanging out at the Neighbor's Club after school. Of course, she'd joined the Neighbor's Club the following day, which could've been his excuse to back out of it, but he wasn't one to break his promises, especially not his promises to his sister.
Kodaka entered the house to see the light was off. Kobato must have been playing one of her vampire shenanigans again. She liked to pretend she was a vampire named Reisys Vi Felicity Sumeragi after watching an anime and reading the manga for it too. A lot of people say children aren't affected by the media they watch, but Kodaka wasn't convinced after seeing how Kobato acts nowadays.
He walked over to the lamp to turn it on. It only made a clicking sound. Damn thing must've gone out, he pondered. He then sighed and headed to the kitchen, which still had its light on. Kobato must've been in there. He entered and saw a bunch of tomato juice on the floor.
"Goddamn it, Kobato, can you at least clean up your messes after you-!" he paused, after having taken a couple steps forward. He saw Kobato, laying in a pool of her own blood - not tomato juice. Blood. She had what appeared to be a sharp wooden stake jammed in her back - in the chest area. He screamed, backing away quickly, about ready to puke. Tears streamed down his face. He quickly ran over to the phone and picked it up, but what he heard next made him freeze.
"Onii-Chan… I killed the vampire," Kodaka slowly turned around, not hitting a single number on the phone yet. What he saw was a smiling little girl. She had long white hair and wore a nun's uniform. Streaming down her own face was the same color liquid Kobato was laying in. On her face was a smile. It couldn't be… Maria!? "I killed that creature of the night. There was a lot of blood that hit me, which makes me kind of regret doing it. This habit doesn't wash too easily when stained red, after all." Maria's smile looked scarier than it normally would considering she was so happy after she'd just ended his sister's life. She seriously took that vampire thing Kobato did way too seriously. "Now, you and I can be together forever, Onii-Chan."
Holding back the urge to vomit, Kodaka coughed and said: "No, uh, Maria… I'm gonna have to call an ambulance…"
"Oh? Why - Why might that be, Onii-Chan?"
"You stabbed my sister! If I don't call them, she - she might-"
"Oh, no worries about that, Onii-Chan. She died way before you came home," Maria explained.
At this point, Kodaka couldn't refrain from getting sick. Though not much, a puddle of vomit landed on the floor.
"I see you liked that vampire a lot, Onii-Chan," Maria commented. She held up a butcher's knife suddenly. "I was afraid this would happen. I guess she turned you into one of her kind.
Kodaka's eyes opened wide in fear. He took two steps back. "Maria… what are you doing with - with that?" he questioned.
"Oh, don't worry, Onii-Chan," Maria explained. "You will still be together with your little sister forever-" Maria was referring to herself in that sentence, not Kobato. "-in Hell!" She charged toward him, the blade of the knife pointing forward.
A few days later, local authorities found three corpses in that household.