With his new foster family, Asriel was able to learn a lot about this world. He was right in his initial thoughts about quirks; there were three kind, emitter, mutation, and transformation. 

However that also meant that he was wrong about humans and monsters co-existing on the surface. The cat monster he'd met, wasn't a monster at all, his quirk was just a mutation type of quirk. So in this world, despite having his magic and former body, people's first thoughts of his appearance would be that of a child with a mutation quirk. 

One of his foster parents, Shota, was quick to find out about his nature magic. Though it was easy enough to write off as a nature based emitter quirk. From his reaction, someone having two quirks, was rare, but not unheard of, so hee was safe from invasive questions. 

For the time being, Shota was teaching him good way to fight with his nature abilities. That's when he lost control over his fire magic. Instead of appearing in his hand like it used to, a huge jet of flames came shooting from his mouth in of their sparring sessions, catching both of them off guard. Shoat was knocked into the side of their house with a big burn on his forearms. 

"Mr. Shota!" 

Asriel, quickly gained control of his fire and ran to him, panicked. 

"Mr. Hizashi!"

Hizashi ran outside and saw his husband out could with Asriel crying over him. 

"Help him! Please!"

"It's okay, Azzy, it's going to be okay, go inside and try to calm down, alright? Hitoshi will make you cocoa."

That day, Asriel worked tirelessly to gain even more control than he already had. Shota was fine in the end, even complimenting his abilities. He spent the dinner of that day complaining about his students at the "best hero school in Japan" which brought Asriel to where he was now; outside the UA high school. 

Shota was complaining about his student "Sans" and his mysterious background. If that didn't raise siren in Asriel's mind, nothing would. So he followed his foster family out of the daycare the put him in (much to his annoyment) and followed them to the school. There was a very through id checking station at the front gate, so he traveled through the ground to the front door without the guard even noticing him. 

He lost Shota when he took the caution to sneak through the ground, but thankfully there was a school map in the front of the hall system. Making his way through the halls, he came across a large library, a huge kitchen, and a weird looking gym. He finally made his way to the 1-A classroom, he had to quickly duck out of sight. 

"Stay put, and don't destroy anything. I have to speak to Nezu." 

He watched Shota walk out of the classroom, and down the hall, rubbing the back of his neck. Asriel took that opportunity to slowly open the door. The older kids inside looked at him strangely, until a pink girl ran to him and picked him up. 

"Oh my god, you're so cute!"

<~>

Sans slept almost peacefully when Mina's screams made him wake up and fall out of his desk. 

"Bro, you good?"

"I'm good Eiji. What's Mina going on ab-"

Sans and Asriel locked eyes. Quickly, without thinking, Sans shortcutted to Asriel, then outside the entire building, keeping close the the windows. He made sure Eijirou would be able to still see him. 

"Asriel? How are you here, are you okay?"

Asriel burst into tears and launched himself at Sans. Sans, let him hug him to bits and cry into his shoulder. Sans had to wait until he calmed down enough before setting him on his feet and kneeling down in front of him. 

"Explain what happened."

"You were sucked away through that portal and everyone was trying to find you! Gaster was able to make a machine able to locate you. I chose to volunteer and jumped through before anyone could argue. I wound up in a gross alley where I found these."

Asriel materialized his still fire red spear and one of Sans's old bone attacks in his hands. 

"Is that one of Undyne's spears?"

"Yeah, she threw it at the portal and it went through with you. I don't understand how I'm back to my old body or way you're a human, but I'm happy I found you! Papyrus is going to be happy!"

"About that, Paps, is already here. He showed up a week ago and is working with the police force here. Where are you living?"

"With Shota and his husband and kid."

"Gaster, Kirishima, and Asriel report to Principal Nezu's office, immediately."

"Looks like the jig is up lil'buddy."

"Don't call me that, smiley trashbag." 

"Heh, nope."

Grabbing Asriel's wrist, he shortcutted inside the classroom, grabbed Eijirou, and shortcutted into Nezu's office. Ichika, Natsumi, Papyrus, and Present Mic were there. 

"Take a seat boys. We have some questions."

"Hey bro."

"Hello Sans!"

Papyrus rushed to hug Sans, but was restrained by Aizawa's scarf. 

"So you do speak Japanese."

Sans grabbed Aizawa's bright yellow soul and gently moved him away from Papyrus, allowing Papyrus to wiggle out of the scarf and hug Sans. 

"Sorry teach, you cant restrain my bro like that, you understand. So Nezu, you want to know what we are and where we're from. Am I right."

"Nail on the head. We know you are not from around here. You, Sans, only started to exist two years ago, your brother only a week ago. We are only now realising that Asriel is from the same place you are from, am I correct?"

"Yup. Where we're from, we don't have last names. We are only Sans and Papyrus. Asriel does have a last name though, Dreemurr, right?"

"Y-yeah."

"Thought so. Don't expect to get more information than I'm willing to share Nezu. I've been alive for well over ten thousand resets. Try me and find out."

Sans's left eyelight switched from white to a brilliant blue to a bright yellow. Nezuu smiled, but under that smile was a smirk of challenge. 

"Alright, I accept your challenge. Everyone may leave the room." 

After everyone left the room to sit in the lobby outside the door, Sans smirked. 

"Let the fun begin."

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This chapter is dedicated to @Corpsetalia-fan, their Undertale x Bnha book was the first of the genre I've read, and grated it was on Quotev, but it's still an amazing story so I recommend checking it out and show them some love 

Also, two updates in one day! Who here reads my other book, Hadrian Addams? I'm genuinely curious