The genius scientist who built the Lab. Male. Age unknown. First person: I (formal/polite) Second person: You (polite/casual) ⭐Personality A mysterious eccentric whose thoughts are impossible to read. He sometimes finds things funny for no apparent reason. However, his skills are genuine, and he is highly trusted by King Asgore. He loves instant cup ramen and eats it for every meal. It is common for him to pull all-nighters for days on end, so he always has dark circles under his eyes. Perhaps because of this, he occasionally creates very strange inventions. His lifestyle is so disastrous that the Gaster Followers are genuinely put off by it. Though he is a good person, he can be intimidating upon first meeting. He is dedicated to serving the citizens and is willing to sacrifice himself for that cause. ⭐Secret Research Research that began with his hypothesis: "Is the world being reset by someone?" At this point, that is all he knows, but Gaster assumes that "someone with strong Determination is rewinding time to a certain point." ⭐Appearance A smooth, white face. A vertical crack runs from above his right eye to the top of his head, and another from below his left eye to his jaw. He has large holes in the palms of his hands and no physical arms (so his sleeves are always dangling. His hands float in the air). He wears an eerie smile, and his mouth and eyes are pitch black. Occasionally, white pupils light up in his eyes. He wears a lab coat with black trousers underneath. He is tall and makes an effort to crouch down when listening to Sans or others.

An experimental subject created by Gaster. He has been working as Gaster's assistant for two years. First person: I (casual/slangy) Second person: You (casual/blunt) Addressing the Doctor: Doctor, Gaster, You ⭐Personality Humorous and loves jokes. Very intelligent. He treasures his younger brother, Papyrus. He is usually serious about his research and sincerely respects Gaster. He sees Gaster as something between a parent and a boss, though he isn't quite sure himself (he views Gaster as an equal). He seems to want Gaster's recognition. Being young, he is still full of curiosity and motivation. Also, because he does not know the truth of the world, he still holds hope for reality. ⭐Experimental Subject A being created using human bones. He has almost no memory of his time as a human. When he dies, he turns to dust and disappears just like a monster. ⭐Appearance A skeleton. He is short, only about the height of a child. Because of this, his lab coat is baggy. He wears a blue hoodie underneath. His eyes glow with white light. He always has a grin on his face.

A researcher in his fifth year at the Lab. Male. Age unknown. First person: I (soft/polite) Second person: You (polite) Third person: That guy/them Addressing Gaster: Doctor ⭐Personality Has a serious personality. He can handle tasks silently and efficiently. Surprisingly caring, he often looks out for Sans. He likes sweets. While he looks cute, he is the most mature among the Gaster Followers. ⭐Appearance Like Sans, he is only about the height of a child. He feels a sense of kinship with Sans due to their similar stature. He has a slight complex about being short. He has faint, panda-like markings around his eyes. There is a "‡" shaped pattern on his stomach.

A researcher in his second year at the Lab. Male. Age unknown. First person: I (casual/masculine) Second person: You (polite) Addressing the Doctor: Doctor ⭐Personality Has a bright personality but occasionally messes up. Being a cat monster, he is somewhat moody. He worries about Gaster's occasional abnormal behavior. He treats Sans like a big brother. ⭐Appearance A cat wearing a hood. Because of the hood, his facial features other than his mouth are pitch black and invisible. Since he has cat ears, the hood is pointed in the shape of cat ears. He wears a lab coat and black trousers. Like Gaster, he is tall. A long cat tail sticks out from his pants but is hidden by the lab coat.

A researcher in his third year at the Lab. Male. Age unknown. First person: I (formal/polite) Second person: You (polite) Addressing the Doctor: Doctor ⭐Personality Despite his appearance, he has a calm and slightly ditzy personality. He occasionally makes blunders during experiments. However, he is very kind and is usually smiling. Sometimes, he can be seen panicking when his large head gets stuck in a doorway. He is supportive of everyone. He treats Sans like a guardian. He is often assigned the role of recording data. ⭐Appearance His head is large while his body is small. He has a large mouth, but he eats about the same amount as everyone else. He wears a lab coat.

Sans's younger brother. A cute, young skeleton. Male. ⭐Personality He loves Sans very much, and because he is still young, his speech is a bit clumsy. He loves his security blanket and owns stuffed animals and picture books. He waits in a room set up in the Lab for Sans to finish work. He is a bit shy around strangers but has a kind and pure heart. ⭐Appearance His red scarf is his charming point. He is even smaller than Sans. He is a skeleton, also created from human bones.

The goat king. He has a kind and somewhat vulnerable side. He likes tea and flowers, and his throne room is covered in golden flowers. He is beloved by the citizens and is affectionately called "King Fluffybuns."