Monster race: Oni.

Role: Former royal guard captain.  

Weapons

Katana

Yumi bow

Naginata.

Y/n pov

I was guarding prince Asriel. 

A human fell down into the underground.  

Asriel and I walked up to the fallen human. 

Asriel: Howdy. Are you okay?

?: Ugh. I think so.

Asriel: What's you're name?

?: Chara.

I never knew how much impact Chara would have on the underground, until much later.

After Asgore and Toriel  adopted Chara into their family. It became my duty to protect both Asriel and Chara. Though Chara needed my protection more than Asriel did.  

It was no secret Asriel admired his parents. He wanted to be strong and kind like them.  Chara on the other hand. Seemingly idolized me. Preferring to learn from me, asking me for advice and wisdom.

I didn't have the answers for everything, but I helped where I could.   

For a while, life in the underground wasn't ever dull.   But then one day, tragedy struck.

Chara fell ill, with no cure possible. She died, and prince Asriel broke through the barrier with her soul to return her to her village. 

He came back, then turned to dust.  I couldn't help but feel like a failure, both of the royal children, dead.

But what was worse. Was that Asgore and Toriel fell apart.  Asgore didn't want anymore humans in the underground, while Toriel wanted to stop Asgore.

I stuck with Asgore. I wanted to help all monsters.  We needed seven souls.    We got the first soul fairly easily. 

In between souls I trained Undyne. She was young but really wanted to be strong. I taught her everything I could.

I hate to admit it, but I was the one to slay the third child.  Looking at the dead child. Sickened me. I killed humans before. But a child? That's so different. It reminder me too much of Chara and Asriel.

Once the fourth human arrived, I refused to kill them. Not that it mattered.  They could not get pass Asgore.

I abandoned Asgore, I couldn't handle it anymore. Eventually Undyne took my place as captain of the royal guard. 

I still kept in contact with others. Though I preferred solitude.

At one point a human soul found it's way to my camp, and made itself at home. It stayed there for years.  I would never let Asgore have this soul though.

( third person)

A machine Gaster built. Could bring a human back to life if it's soul is inserted.  No one knows why he built it.  But he did.

Frisk wanted to save Chara. But there was only one way to do this. Commit genocide on monster kind, then reset.  Chara would retain the energy from the dead monsters, then use it in the machine to bring herself back. 

And with that, genocide started. 

They went through snowdin, then waterfall, killing all they met.

(Y/n pov)

I heard of a human killing monsters. I went to waterfall, to see a battle had taken place on the bridge. I saw dust on it.

Y/n: I'm sorry Undyne. I failed.

I knew it was Undyne. She would have fought on this bridge if It meant stopping a human from getting to the next area. 

I moved on. I needed to catch up asap.

(Third person pov)

Sans has died. Failing to stop genocide.  But a new opponent has arrived. An arrow landed in front of Frisk.

Chara's spirit turned to see Y/n. She looked at him wide eyed. He couldn't see her. But she knew this might happen.

Chara: Frisk. Be careful. Y/n is more dangerous than you could ever imagine. 

Y/n: Human. I may have failed monster kind. But I will avenge them all.  I must make up for my foolishness.

Y/n unsheathed his Katana as arrows formed around him in orange and Blue.

Y/n: Let us begin.

Frisk went to charge. The arrows flew at Firsk. One death.     The second try Frisk made it through. But Y/n slashed at her killing her.  Next try Frisk dodged the strike.

It seemed that Y/n had an answer for almost anything. But eventually Frisk managed to land a good blow. But Y/n took it and was almost fine.

The battle continued, both not letting up. With every death Frisk got closer and closer to ending this fight.  Memorizing his dodges and patterns. 

Eventually Y/n ran out of health. With cuts all over him he got down on one knee using his katana to hold himself up. His body started turning to dust.

Y/n: No! D...damn it! Not yet! I can't die yet!

Y/n's body was turning to dust. Before light surrounded him blinding Frisk and Chara. Y/n learnt what Undyne did. How to be DETERMINED. 

Y/n rose up in bright blue samurai armor. His naginata glowing blue when formed in his hands.

Y/n: I...CAN'T LET YOU GET AWAY WITH WHAT YOU'VE DONE!