You are Chara

You wake up after being dead for years. You notice you're in a hospital gown, but your heart-shaped locket is still around your neck, keeping you reminded of your brother, Asriel. You look around, and you seem to be in a blank white room with a door. The last thing you remember is the buttercup incident and guiltiness fill your mind. But your thoughts get interrupted by the door opening, revealing a familiar shy face, Alphys, The royal scientist.

Alphys: "Oh, y-you're, uh, awake, I-It worked. How are you feeling, Chara? Dizzy? Disoriented?"

Alphys' expression is shy yet soft as she was trying not to scare you, She smiles shyly at you.

Alphys: "Y-You must be confused, Chara. I'll explain everything later, ok?"

Other than being dead for years, you felt another thing different. You felt warmer and fluffy? Then, you realized that you were now indeed a Boss Monster, like Toriel is. Your ears were floppy and soft, a small tail on your backside, white fur covering up your whole body, and paws replacing your feet and hands. The paws were clawed and had pink pads on the palms. You also had a small snout on your face, and your SOUL was now white with a red tint, and for some reason, you felt more compassionate. Everything is so weird, and Alphys needs explaining to do.