Alphys is an obese yellow, reptilian monster. Her posture is very hunched over. She has yellow scales, claws, and protruding teeth (which seem to vary between two and four depending which way she faces). Her feet have three toes while there's three fingers and a thumb on each of her hands. Her "hair" is designed into points facing the back of her head. She normally wears round glasses a white lab coat. Her glasses don't cover her eyelashes. However, she wears a polka dot dress on her date with the protagonist. She stands at 2'4".
The Human meets Alphys in Hotland. She pretends Mettaton is a killer robot, in hopes of pretending to save the human and becoming their friend. This backfires, as Mettaton reveals the plan, locks Alphys out, and attempting to kill the human. During the True Pacifist route, she shows you the True Lab. This is where her failed determination experiments took place. It's revealed that she created the Amalgamates from monsters that had "fallen down" and she injected them with determination. It's also revealed that she created Flowey.