Alphys (/ˈælfiz, ˈælfɪs/ ALF-eez, ALF-iss) is a character in the 2015 video game Undertale and the 2018 video game Deltarune. She is a bisexual reptilian monster who serves as the royal scientist under the leader of the Underground, Asgore, and is a fan of anime and manga. She is friends with Mettaton, a robot, and the captain of the Royal Guard, Undyne, who in the True Pacifist route becomes her girlfriend. She lives with social anxiety and depression, and the depiction of her struggles with these have been the subject of praise and commentary by critics. She watches the player-character throughout the game, offering guidance at some points, though with limited benefit. She is revealed to have dark secrets, including ones that relate to experimentation. She also appears in Deltarune as a school teacher.

Alphys was designed by Toby Fox and Temmie Chang. She was initially conceived as a male character, but Fox did not like this, so he made minor changes to her design and changed her gender. She has received generally positive reception, though some critics identified her as an annoying character. She has been identified as a positive representation of both neurodivergent and LGBTQ+ characters, with multiple critics identifying her as a relatable character. Alphys overcoming her issues was particularly cited as a strong example of Undertale's message of characters finding courage in themselves by Destructoid writer CJ Andriessen.

Alphys was created by Toby Fox for the video game Undertale, with collaboration from other artists, including Temmie Chang. She is a bisexual yellow lizard monster who typically wears a lab coat and glasses, working as a scientist. She is implied to have multiple mental illnesses. She is also a fan of anime and manga. Temmie created sprite art for her in Microsoft Paint, and Fox changed her eyes to make her cuter. An initial concept art based on the sprite was made by Temmie, though Fox felt it was too round and that she should be more "triangular." She has the most faces of any character in the game.

Alphys first appeared in the 2015 video game Undertale. She is the royal scientist working under the king of the Underground, Asgore, researching how to break through the barrier using human souls. She is also close friends with the captain of the Royal Guard, Undyne, having met her while hanging out in the garbage dump. When the human meets Alphys, she guides them through Hotland, though a robot she said she created, Mettaton, attacks the human at various points. She offers to help with puzzles over the upgraded phone she gives the human with limited results, though they also get social media updates from her. They discover from Mettaton that she created the puzzles to involve herself in the human's quest.

Her fate is dependent on the player's actions before the fight against Asgore. In a run where the player does not kill any monsters, Undyne will ask the human to deliver a love letter to her, leading to Alphys misunderstanding and going on a date with the human. Ultimately, she admits her feelings for Undyne, only for Undyne to show up and for them to talk out their feelings. Afterward, the player visits her True Lab, discovering experimentation she conducted on monsters who died to keep them alive by injecting them with "Determination" derived from six human souls, which leads to their bodies fusing together. She ultimately vows to return them to their families and come clean. When attempting the fight against Asgore again, it is interrupted by her and others, only for Flowey to take the monsters' souls, becoming Asriel Dreemurr. She appears in the fight against him, and must be saved by the human by reminding her of who she is. After the fight, she, along with other monsters, are able to escape through the now-destroyed Barrier. Alphys is seen in the credits being kissed by Undyne.

She later appears in Deltarune as a school teacher, with this depiction not being a continuation of her story in Undertale.

Alphys has received multiple pieces of merchandise, multiple pieces produced by Fangamer. She was included in the second set of "Little Buddies" figures produced in collaboration with Fangamer. Fangamer also produced a keychain plush of Alphys, although it stopped being sold as of January 30 2024.^{[failed verification]} A soundtrack compiling music related to Mettaton and Alphys was also produced by Fangamer, as well as being featured in sets of stickers and pins.

Since appearing in Undertale, Alphys has received generally positive reception, though some critics found her annoying. Game Informer writer Daniel Tack and Mike Fatter were both critical of Alphys; Tack found Alphys annoying, while Fatter felt that Alphys was not well written and disrupted the game's flow. Kotaku writer Jason Schreier noted that he could not stand Alphys' "nerdery," stating that Undyne could do better than her, but the final boss fight helped him appreciate her somewhat. Designer and composer Laura Shigihara, while discussing her enjoyment of games making players feel like they are "part of its culture," noted Alphys and her social media updates as an example of this, saying that it made both her and the world of Undertale feel "very real."