So before we start let me tell you this is a non ship fic for Saiki. It's more like a revival short story about friendship more than romantic relationship.
It is also based off an old AU idea I had of necromancer Shun.
It also works with trans Saiki, which is canon.

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Our story starts when Shun is 8 and the family dog runs out into the road. He doesn't know how when he stops to think about it. He just remembers his baby brother and dog going out to play, then popping his head out to seeing the car hit it.
His brother screams running to their mother leaving Shun to go pick up the poor thing. He slowly takes them into his arms sniffling as he wraps their tiny body up with his own coat to keep his brother from being scared for life.
Shun then sits on the lawn sniffling cradling the dog in his arms. Yet, as he mourns he finds he cut himself on something. Maybe it was the dog's claw. Maybe it was on the road. All he knows it's that his blood dropped on the dogs nose...and suddenly he could feel it's body shifting.
The dogs body shifted back to the way it was before and it wide awake. Just like that he preforms his only revival as his mother comes rushing out. Thankfully when the dog is brought back, sure with a broken leg, but he brought him back. He brought back their dog.
A dog that lived for another 5 years until turning 15 and dying peacefully in its sleep.
He's also revived bugs before. Right after this he found a smushed butterfly and got curious if maybe his dog wasn't really dead...maybe it was just his mind playing tricks on him.
So he pricks his finger and drops some of his blood on the smushed butterfly. The butterfly pops back into its lively form, flying off. He's resurrected a butterfly.
And from there he starts to convince himself that he's the jet black wing, just like in the manga. He's determined to prove that he's got this power for a reason. He has to have them for a reason.
....which comes back to bite him in the butt one day in high school.
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So to put it simply ...shun messed up. Let's just say that.

To be honest he should have thought this through more. All he knew was that his friends asked him to come to a Halloween party, and he was determined to prove he was the jet black wing once there. They had been joking about the powers he had told them he had.

"No Shin I can get most of it. Like the magic hand thing I could see how that pops into your head." Aren sighs looking at him. Shun blushed looking down at the campfire as his few friends talked around him about him.
"Yeah, and we can see how you got the whole magic bought thing.." Chisato says leaning back holding a drink in her hand.
"Look I can do the stu-
"We're not saying you can't we just go a big question.Like how did you get necromancy out of it?" Nedō asks looking genuinely curious.
"What do you mean? I've done it before."
"Suuuuureee you have." Aren sighs.
"No I mean it! My brothers dog go hit by a car and I brought him back! And I brought bugs back before! I mean it!" Shun says determined, mainly cause this is on the one thing he knows he can do. He has done it before. It's the one thing that's made him determined to prove he's special.
"We know it's just kinda hard to believe."
"Oh and Reita talking to ghost is easier to believe!" Shun blurts out glaring at the purple haired boy.
"Yeah well I don't claim to be able to bring them back!"
By now other students are seeing the argument and have stopped to watch.
"Look we aren't-"
"NO, NO I am sick of being to butt of the joke! I will prove it to you! I will revive something for you guys! Name anything! Name a single dead thing and I will revive it!" He yells determined to prove himself. He's done with this all. He's gonna prove he's special and not a liar.
"Shun you don't have to. We just wanna know where the idea came from." Aren sighs holding his hand trying to be supportive. To him this is just his friend trying to find a way to explain why he was so ignored as a child.
"No I'll do it! Name one thing! One thing an I will bring it back!"
"Oh we could send him the the corpse forest!" Saiko suggested from the watching crowd. Everyone just looks seeming pale.

You see the corpse forest is an old forest where people have said to been buried after being murdered for years. So they are told not to go there under any circumstance. Like at all.

So yeah, Shun goes with his friends to the forest to prove he cane revive a body. From here things are gonna spiral.

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Once in the forest the classmates keep looking for a dead thing. And for once they find nothing but a creepy forest.

After a while they go to give up when they find a branch that Nendo is certain is a hand. Yet, to everyone else it looks like an old withered branch. It looks like it has been through hell and is ready to give out. Not to mention Reita hasn't seen a single ghost here so this isn't a dead person.

Regardless of this they have Shun probe his powers. They have him prick his finger on a rock and have a drop of his blood fall onto the corpse. Fro there they wait...some classmates to tell jokes and his friends to be there for emotional support when it doesn't work again.

So they wait
And wait
And wait.
Nothing happens so the jokes dusty coming from the classmates. Ones about the powerful jet black and his skills. Ones that make him tear up a bit, quickly rubbing his eyes getting a tear wiped off his face which also lands on his hand...
And that's when it happens.
The branch traces up grabbing the person slides to it, which happens to be Saiko. Everyone screams running off except for Shin and his buds. At first some start to try and pry the hands off of him in a blind panic. Then suddenly Shun drops to his knees and starts digging the hand up. No one clues into why until the digging brings up another hand that's been trying to free itself.
That's when those who stuck around realize Shun brought back a person. He brought back a dead person who was currently suffocating in the ground.
Needless to say they dig with him just as the hand lets go of Shun. That is except for Nendo. He seems to just be holding the hand that's above ground now which he explains had been shaking in fear seconds ago.
As they dig they soon unbury a pink haired corpse who is gasping for breath while their body reforms, while shaking for a second. Once they catch their breath they all just sit around for a second wondering what the hell just happened when Nendo speaks up, still seeming to hold the hand, (that is now clinging to him since the corpse is still scared crap less of what they just experienced no matter how much he denies it) speaks up.
  "So we're all buddies with a necromancy..pretty cool..."

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The body Shun brought back belonged to Saiki, a young Psyche from the 1800's, who was killed by a supposed suitor for identify different from his birth gender. A suitor who may or may not have stole Saikis money when he was killed, and may have buried him alive without telling any one of their families. And by bringing him back, Shun has effectively given him a second chance at life..a life with enhanced powers.

Turns out once when a Psychic is revived their powers just jump up and get better. Shun doesn't know how gut all he knows for sure is when Saiki first described his power to read minds and float an object in the air, he definitely didn't mention being able to teleport.

Of course they learn this together when Shun moves Saiki into his father's old abandoned work room which hasn't been used for year since his dad left. And it takes so readjusting for the both of them. Shun, to have a person in his life who was picked on for being special too and having someone who acts more like a father than his ever did in his life. Saiki, for living in a more accepting time and with new powers seeming to pop up every other day now.
Thankful Shun's friends are there to help keep the peace and keep everyone happy...and in Nendō's case get some new people to try new restaurants with.
Either way they're happy he's around, and happy to have a new friend.
(This idea may be built upon more later down the line lol)