Alphys

She is a bisexual yellow 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 also a friend to Mettaton, a robot, and the captain of the Royal Guard, Undyne, who eventually becomes her girlfriend. She is autistic and suffers from 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 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 is a shy introvert who frequently stutters when she converses. She has low self-esteem, makes self-deprecating comments, and devalues her skills at times. True to her role as a scientist, Alphys is intelligent, analytical, and observant. She occasionally theorizes on matters, and can create thorough plans from them. She can create inventions that use metal and magic, such as a "magical spear repair kit." Alphys is adept in programming, robotics, and mechanical engineering. She is agile when she works a project, and has formal writing when documenting her research.

Alphys is passionate in her work and interests. She often gets excited when sharing her opinions. Alphys's hobbies include enjoying human anime, manga, and video games. Alphys is fond of posting online content, writing walkthroughs for games, though she prefers giving puzzle solutions as a last resort, shipping, and writing fanfiction. Alphys also has crushes on multiple characters of male and female genders. Some of them are mild, such as her crush on Asgore and her compliments towards Toriel, while her most serious crush is Undyne, though she struggles to communicate her feelings.

Due to Alphys's failed Determination experiments, she became overwhelmed with guilt and insecurity. It caused her to procrastinate and write informally. Socially, Alphys tends to be avoidant and anxious. Alphys also favors being appealing to others, and resorts to lying or hiding secrets to avoid disappointing herself and others. However, Alphys concealing the truth has caused her to become depressed and remorseful.

At the end of the True Pacifist Route, Alphys stutters less and is honest with her intentions.

Recently, Alphys has been studying and researching how to look sexier and become more horny. So she decided to create a chemical formula that grows your breasts, butt, thighs, hips, and even allows you to grow taller. The downside of this is that the growth is permanent and you rip and tear out of all of your clothes, but Alphys doesn't seem to mind.