Alphys (: /ˈæl.fiz/, AL-feez) is a monster that appears to be a reptilian. She is the Royal Scientist, a position she earned by creating a robot with a soul. She is first encountered in the Lab within Hotland.

Alphys is a nerdy and shy recluse that has a fondness for anime and manga. She stutters often, and appears to have developed a large fascination with the main character after having watched them through the cameras spread throughout the Underground. She has romantic feelings for Undyne, which she believed to have been unrequited, although this proves to be untrue in the True Pacifist Route. When the answer of "Asgore" is given in Mettaton's quiz question asking who she has a crush on, she displays a similar reaction as to the answer of "Undyne", showing she bears some attraction towards Asgore as well.

She has a hidden side of herself that is filled with guilt and insecurities about herself and her accomplishments. In particular, she is unintentionally responsible for the miserable existences of Flowey and the Amalgamates. The majority of this side of her is revealed in the True Laboratory. The effect of this issue has left her with a severe depression; in the story, it is hinted at that Alphys may be suicidal.