Passion of Injustice - Undyne x (Female)Reader

Regret

In the blackness of sleep, the sweet, sad melody began to change. Something new and haunting in a different way struck a fresher chord of emotion within your soul - some desperate, determined feeling, as though it were an undying soldier's last breath of air, or that final attempt at striking the enemy down.
Yes...there was a great sadness in those notes.
You recognize the change of sound from a music box to a piano, and that's when you see her face form out of the blackness. You see the very woman you had left behind, in all her deepest, darkest fury of mourning.

You begin to stir, but it doesn't feel like you're on cold rock anymore.
Instead, you feel cushioned - like your body is encased in softness and warmth.
The strong, earthy smell of damp mold is gone. Instead, you smell linen and other cozy fragrances.

The piano keys pound louder into your ears, and your eyes flash open. You are alone in someone's queen sized bed.
The room is very dimly lit, but you recognize your surroundings the instant you fling your gaze around; anime posters, swords on display - that infamous suit of armor you remember so well, stacked together on a mannequin.

Well, guess Undyne had found you, eh?

You glance over at a desk beside the bed. An empty pen is almost ready to roll off the edge, with frustrated claw marks visibly parallel to it. The lamp aimed at the top looks like it hasn't been off for very long.
You notice a small metallic can at the foot of the work area - and see loads upon loads of crumpled papers from some kind of notebook.
The can is so loaded, it's overflowing. Many little balls of paper are scattered around the floor.

Swinging your legs out from under the covers, you sit up. You're still a bit sore from falling asleep on such a hard surface, but you endure.
Your curiosity overcoming you, you bend down to the nearest globe of a notepad page, and pluck it from the ground with your fingers.
You feel uneasy while you pry every individual fold and corner open, anticipating whatever words may jump out at you.

Finally. You read them.

"Human, no, ______."
"I don't know how to tell you I'm sorry for hurting you, but, I am. I don't know how to get you to listen to me. I understand that you probably don't ever want to see me again, but..."
"You have to understand something. When I lost her, I lost a piece of myself that I was never getting back. I've tried filling it with all kinds of things: excessive training, a bit of alcohol, and... now sex."
"Alphys and I were never more than friends. I had always wished that we could have been. I think I cared about her more than anyone else in the whole Underground, and damn did I want her bad."
"Why I never told her? I never could really find the words, no matter how passionate or confident I was. I'd write letters like this one and just crumple them up and throw them away because I felt like they were never good enough for her."
"I regret not having the courage to actually tell her every day of my fucking life, because maybe if I had, she'd still be here."
"Maybe she wouldn't have killed herself if I hadn't been so afraid to love her."
"I didn't want to make the same mistake with you. See, even though I barely know you, I haven't felt this way for anyone since Alphys. Perhaps it's amplified because you're suicidal like she was, but it can't only be that, can it? I mean, even when I first saw you, I thought you were pretty hot - for a human, anyway."
"I don't know if any of what I'm writing is going to make you feel any better, or despise me any less. If anything, you might end up hating me more. At least it helps me get a few things off my chest, which feels...nice."
"Before you throw everything away, perhaps you should just stop and consider how much I suffer because I lost Alphys, and how much more I'd suffer losing you. Gosh, how do I even go on myself after all this pain?"

She hadn't even signed her name before discarding it.

Your eyes are congested with tears of compassion - and regret that you had ever left her. You feel selfish of course, but you decide that you're going to make it right.
You know she's out there, thrashing away at her piano the same way her heart has been thrashed around for God only knows how long.

Trembling a little with anxiety, you begin to make your way towards her bedroom door. Your knees are wobbly and your breath shallow.
Similarly to how she seemed to stuggle with words, your own mind has become one huge jumble of emotions that you can't describe, as you try to construct them properly in a sentence before you even have to open your mouth.

The door has cracked open, and there she is.
Her back is turned to you of course, her shoulders slumped forward and her head nearly invisible behind them. Her muscular arms just won't stop furiously gliding around the keys, and you stare and stare at the battle scars that decorate them.

You speak at last.
"H-Hello, Undyne..."

She stops playing. In fact, not only has she stopped playing, but she's completely frozen in place. She won't even turn her head around to greet you.
The room has gone deathly still and uncomfortably silent.
You are not happy with this.

Fighting your terror, you inch a few steps closer.
"S-so, um...are - are you alright?"

Her reply is a deep growl.
"Am I alright?"

At last, she angrily spins around in her chair to face you.
Her yellow eyes are still a little puffy and her scowl is deep.

"Am...I...alright? You've got some nerve asking me that, kid."

Your heart sinks at her tone and your stuttering gets worse.
"I-I-I'm s-sorry, Undyne...I know that I - said some hurtful things and...I reacted poorly! I got...caught up in my own feelings again..."

She practically flies out of her seat, charges at you, and then pins you up against the door - slamming it shut and grabbing your shirt in her fist.
Your heart is in a fluttering panic now.

"Yeah, you fucking think?! Apparently in your little messed up world, it's okay to only consider your feelings and to completely ignore the feelings of the people that love your damn guts!"
"You think I just spare the ass of every human twerp who falls down here? You think that my mercy is that fucking cheap when my entire race of people and my King need me to be anything but merciful?!"
"Well forget it! I ought to tear you to shreds right now, punk!"

Your cheeks have become home to a river of tears, as your soul breaks a little.
"I know, I know, it's all my fault! It's always been my own trashy fault! You deserve better than this!"

"Don't you fucking say that!"
"Don't you ever, ever say that, you whining bitch! I don't care what you think of yourself anymore, I'm not going to tolerate this kind of bullshit!"
"I saved your damn, pitiful life - and like Hell you're just gonna walk out on me and get rid of it!"
"Now you're going to stay here and stay alive, or I'll give you something to really cry about!"

Your voice chokes out. You can't say anything in reply.
Instead, you just nod in defeat.

She releases your shirt, dropping you, and then hits you with the door as she storms into her room, once again slamming it shut.
You don't really care about the pain.

All you can do is crawl into the nearest corner, fold into the fetal position, and finish off your weeping.

...