Blue Exorcist: 'Little Sister?'

Chapter 1 Just another normal day

"RIN! WAKE UP!"
Rin groaned as he rubbed his eyes and saw Kuro sitting in his chest pawing at him. Kuro watched him with wide eyes as Rin finally looked at Kuro with a bit of anger and tiredness. "What, Kuro?" He mumbled sleepily, still too tired to get up.
"Don't you have class?" Kuro said. Kuro wasn't a normal cat, he was a demon cat. Yukio, Rin's younger twin brother, told True Cross Academy that Kuro was his familiar. Rin was still tired after what happened a few days ago with Gehenna Gate and his Father, Satan.
What, do you want me to summarize what happened? Alright, I guess I will. Rin and Yukio had to close Gehenna Gate because it was opened by their crazy grandfather who wanted to destroy Gehenna. Which in the end, majorly failed. Satan ended up unlocking Yukio's demon powers and taking over his body and creating a new world. With the help of their friends, such as Shiemi, Shura, Izumo, Shima, Suguro, they were able to successfully close the gate. There, now you know what happened.
Now, back to the story. After Kuro reminded Rin he had class, Rin jumped up and out of bed rushing to check the time. "Shit!" He exclaimed. "I'm gonna be late!" Rin pulled his clothes on sloppily letting his tail hang out and grabbed all of his supplies and rushed out of the Boys' Dormitory.

...

"Now class, I would like you to open your books to page 134." Yukio, or Mr. Okumura (which is what the students call him), called out and the students began opening their books. He was just about to say something when Rin burst into the room and rushed to his seat.
"Rin," Yukio said and glared at him through his glasses. Rin, still trying to catch his breath from sprinting to class, looked up at Yukio huffing and puffing. "Heh-uh-present!" Rin said nervously. The entire class groaned. "You're late as usual, Rin." Yukio spoke with a calm and serious tone towards his older brother.
"Yeah, yeah, I know. Now what page are we on?" Rin peeked at Shiemi's book, since she sat right beside him, and flipped open to the page that the class was on. Yukio sighed and continued on with his lesson. Rin tried to pay attention, but ended up falling asleep.
"Rin?" Shiemi whispered and poked his shoulder. Rin woke up with a start and looked over at Shiemi. "You fell asleep again." She said and turned his page to the one they were on next. "O-oh, heh heh, I didn't get much sleep last night." Rin explained nervously then turned to his book to hide his blushing. Shiemi smiled then turned back to her book.
"And that will be all for today class. Your homework is to read the next two sections and take notes. You're dismissed." Yukio said as he wrote the assignment on the board as he spoke. After everyone wrote down what they needed to do, they headed to their next class.
"Hey, Yukio?" Rin said before walking off. "Hm?" Yukio turned to Rin questioningly. "What is it?" Rin looked at Yukio with a serious look. "I've been feeling weird after what happened with the Gehenna Gate. It's like something like a stomach pain where I'm about to throw up but I can't. Do you feel the same way?"
Yukio paused and then nodded. Rin looked at his stomach then up at Yukio. "What do you think it means?" Yukio sighed and shrugged. "I've been trying to figure out what it means, but I don't know. Just head to your next class, and if it gets worse, we'll go to the dormitory." Rin gave a nod then headed out.