So, this is going to be kind of supernatural-y/magic AU.
3rd pov
Karma would like it to go in the records that he was not a willing participant in this. He was not and still is not. You see, Karma Akabane was "born" on December 25, though exactly what year is still unknown to Karma himself. Karma himself, is a phoenix that had a bad case of wrong time wrong place, and had got the wrong end of a revenge driven fallen angels curse. He had not known the immediate affects - that's what happens what an asshole curses you and it makes you sleep for 100s of years - until he had woken and found it to be at least the early 2000s. He had been asleep and ageless for almost a thousand years.
For a phoenix, most lived naturally long lives, but even they hit their limits around 800. Karma had found out all too quickly what his curse was and he hated it. Still does. Karma was already about 500 years old before the curse was placed on him, and he had stopped physically aging by 18 - he is slightly taller than the average teenager in middle school - and he graduated from a many number of schools, though all under different names. He had been all over the world and has tons of money saved up, which he uses to buy a house in this small little town.
Honestly, he had not planned to go to Kunugigaoka high, but his plans had been pretty quickly derailed and so he makes up an excuse on the spot - he'd been suspended for violence - and gives the officer a date for his return to school. The man nods and accepts that before heading back off to patrol, giving the others a description of Karma so they know when he has to go back to school. So, with a bit of frustration, Karma uses magic to make the kids and adults in the main building fake memories of him and his suspension - no need to bother with grade E, he can tell their all magical even from the bottom of the mountain - and prepares for school to start.
Imagine his surprise when he's tasked with helping to kill their teacher - Korosensei, a curse inflicted on him slowly over time that will cause him to blow up within a year - and he starts to make friends. It's a weird experience, what with witches, wizards, golems, machines, vampires, werewolves, demons and a few angels all mixed together, but Karma enjoys his time and starts to love each of them, holding them dear to his heart. He's lived for almost 2,500 years, he knows when to treasure what he's found and that's exactly what he does. Of course, the rest of the class know he's a supernatural creature, but not what he is or else they might panic, as most people do when they run into a phoenix.
They had been given a bad reputation by a rogue early on in the years when the supernatural were coming out into the open and so most people are weary around them, as if they're going to dive bomb and explode like the asshole who had given the first impression to humans.
It had only been a few months, but the life that had been literally the worst thing ever, dragging by both too fast and far too slow for Karma, had begun to move at a more normal pace, letting him enjoy and treasure these people that would most likely die long before he got the chance to experience the embrace. With his curse - immortality, something no other creature would wish on another, not even vampires the 'supposed' immortal creatures ( they really only lived for about a thousand years, longer than any other creature) would force immortality on another - he would live forever or until someone found a way to undue the curse.
Now you may be thinking because he's a phoenix he could already be considered immortal, since they can be reborn, but they can really only be reborn 3 times before they die for real. Karma had only died once, when he was about 300 to a hunter thirsting for their first kill, no matter whether it was a peaceful monster or not. Karma had been his unlucky victim and he'd bled to death in a forest all alone for hours.
As the days get warmer Karma finds it hard to resist the urge to lay in the sun, absorbing the warmth into himself, enjoying this for hours as well as the gossip of his classmates who are trying to determine what he might be. He finds great amusement in how far off they are - werewolf, golem and an angel - but let's them speculate. The class continues to grow closer and closer, becoming the family that Karma had never truly had - his parents killed when he was pretty young, only about 100 - and he decides they're all his, to protect and cherish and love, from anything and everything if need be.
It happens that he gives them the first clue during winter unintentionally. He had never liked the cold much, though on average it didn't bother him unless it dropped real far down. However, since the E building doesn't have heating, everyone has to come in bundled up in clothes upon clothes and even poor Isogai looks like he's considering buying more clothes - not for himself but his siblings - when Karma decides enough is enough and interferes.
"You can stop wearing all those clothes inside, give me about a week and you won't need them outside either. Promise."
The class stare, but they also trust him, however...
"Won't this give us a hint at what you are as a supernatural creature?"
"I don't mind, as long as you all stop coming in shivering. I'm tired of watching you almost freeze because some jackass on a high horse thinks he can play god with his students."
True to his word, the students come into the classroom with no coats, scarves, gloves or any extra clothes but their uniform and are pleasantly surprised to find the room warm, a little on the toasty side. They realize why when they spot Karma crouched on his desk - literally on the desk - and they can see the heat wafting off him into the room. They all settle in their seats and while Terasaka and a few others look weary, they are not burned by the heat coming off of him at all.
Korosensei, Karasuma-sensei and Bitch-sensei look pleased to find the classroom warmer as well, though by the time they get to the room Karma has settled back into his seat and the heat coming off of him isn't as intense, so it's far less seeable, though it maintains a constant heat anyways. They are all grateful, but even more so when a week later, he comes in with small boxes and hands one to everyone. He had come in late, but he even made some for the teachers - who appreciate it, it's freezing most days - and he stops warming the room. The others look panicked and turn to him, but he gestures to the presents.
They all turn back to them, before Hazama takes the dive, popping the lid off the box and then freezing, staring in surprise at what's inside. She reaches in reverently, hands shaking in awe and love, as she pulls out a little necklace, with a little glowing, pulsing feather imbedded directly in the center, designs around it. Everyone else freezes as well, before opening their own boxes.
They all pull out necklaces with little pulsing feathers imbedded in them, with different designs for all of them, and they can feel the heat radiating off the feathers. They pull them over their heads and are unsurprised to feel the warmth sinking into their bodies, forming a little cocoon of warmth around them that the cold can't penetrate. They all turn to Karma who's smiling at them softly and nod, too amazed and awed to properly thank him. Phoenix feathers are hard to obtain since prejudice is so strong against them they rarely come out of hiding. To get enough to make necklaces for each of them with a feather, Karma had spent a lot of money on this.
He really hadn't - pulling the feathers hadn't even really hurt, he was going to lose the small ones soon anyways - and the only hard part had been shaping the necklaces after heating them. The class enjoy their necklaces, wearing them to school every day, getting weird looks from other people for wearing very little besides their uniform, but they don't care.
They get their second clue during during PE class, Karma had been using mostly smooth moves, but when Karasuma lunges at him and tries to trap him, he backflips smoothly over the other with no problem, showing a flexibility and gracefulness only a few creatures can. The class continue to watch, but they're also debating amongst themselves what he could be with flexibility like that - vampire, demon, griffin - and yet again are so far off he almost bursts out laughing, his shoulders shaking only slightly with his restrained laughter - and only spotted by his teachers - before he focuses on his fight.
They get their last and final clue during the fight that is giving even Korosensei trouble. Kayano is risking everything in order to protect their teacher and while they want to join her, most are frozen by the reality that they would really be nothing more than a burden, even if they tapped into the beasts they are. Except Karma steps forward with a sigh that sounds rather tired as Kayano jumps into the air and they all scream when a tentacle pierces her...or so it seems, but instead a flicker and then she's right in front of them with no wound at all, not even a drop of blood.
The battle stops, because Korosensei may be blocking, but he's in no way currently capable of saving her, even if he dearly wanted to. Shiro and his .2 freeze, surprised that they are at such a disadvantage and were taken by such surprise, but focus to find the one who had moved so fast. The class and Korosensei try to find out as well, since as far as they know none of them can do that...until a light catches all of their attention and they turn to a fire in the middle of the field, held in Karma's hand. Suddenly, it strikes the class what their classmate, their brother is and they are awed by him all the more.
Their phoenix gives them a wink and then he's gone, smashing his burning hand into .2 with no regret as the creatures screams and backs away, though the fire does not go away. Karma flips through the air to remove his momentum before landing on his feet, grinning at Shiro - who's hate filled eyes have locked onto him - and then disappears yet again. To Shiro's surprise, he feels the tentacles he injected into his blood stream disappearing, being removed precisely, and he screams when a fire settles on his neck and burns out the root of his tentacles, leaving him as his normal human self.
"Please, I've fought with sirens stronger than you. And for a phoenix, that .2 is absolutely nothing."
What Karma says is true, for a phoenix gliding for fun can hit 23 Mach per hour, but one that is hunting or fighting can hit about 50 or 60 Mach per hour. Karma starts to beat on .2, giving him no choice but to defend, but it's only when he starts powering up a final attack that Karma stops. Karma looks over the attack and sighs, glad to finally be rid of this damned curse, rolling his shoulders, and then pushing off the ground.
Everybody recoils as light starts to shine from his chest, growing so bright none of them can see, until it dies down. .2 is no longer there, vanquished by whatever Karma did and he himself is wobbling, stumbling towards the class. They rush forward, Korosensei grabbing him once he is in reach. Karma groans and shifts, leaning more against his teacher.
"What was that?"
"One of my more powerful attacks. Leaves me exhausted, but not enough to stop me from doing this."
With a twist, Karma reaches into his chest - no blood or anything, so his spirit - and draws out a small, withering black thing. It jerks and flails in Karma's hand, but to every bodies surprise, Karma gently brings it to his mouth and bites into it, causing it to start writing itself out in the air. They watch as he chooses one word and grabs it, before turning to Korosensei, shoving his hand into his chest apologetically and releasing the blessing there. The rest he puts back into the black tangle and puts up into the sky, and they watch as the beam made for Korosensei barely manages to destroy it before it dies.
"What was that?"
"A curse I was inflicted with over 1000 years ago."
"Why?"
"It was wrong time wrong place. Glad to be rid of it."
"...I thought phoenix only lived to about 800 before they were forced to be reborn."
"They do. Unless cursed with immortality."
"Damn. How old are you?"
"Roughly 2500 years."
"Dear god..."
The class is horrified, but can't help but be glad he was cursed. They rather like him and knowing he might be reborn so they can have him again is something none of them are bothered by. They simply gather closer and head into the classroom, having a giant sleepover, the blurry figure of a giant bird, fire in its feathers and eyes crouched over them and protecting them from anything barely seen by most before they slip into sleep.