Underfell Alphys.

Dr. Alphys is the Royal Scientist of the Underground. She does experiments on everyone (mainly fashion for the main cast), is very confident, and doesn't really have sympathy or empathy for others outside her circle. She also has a crush on Undyne and is already dating her.
She thinks only once and chooses immediately. Fashionista.

She enjoys experimenting, sometimes on living monsters and humans. She has no empathy for anyone outside her social circle. Alphys is almost as arrogant as Asgore.

Depite this, she surprisingly has an adorable laugh.

Alphys is a yellow lizard-like monster with a sharp head shape, white and black hypno glasses, sweat, two nose holes and a mouth with two teeth. She wears a white lab coat and a long shirt which is striped red and black at the uppermost part and black elsewhere else.
Undyne actually gave her those glasses because she saw one in anime.

Alphys deliberately made Mettaton's EX form the least pleasant for him, in order to create the ultimate body, dangerous in both design and power. She lied to him that he would get the body of his dreams if he helped her convince Asgore to let her become the Royal Scientist.

Undyne:
She is the current girlfriend of Undyne, though she doesn't spend that much time with her since she is more focused on her work.

Flowey:
Although Flowey and Alphys don't have a direct relationship, it is depicted that Flowey was (technically) created by Alphys when she injected Determination into Flowey (Alphys is unaware of this).

Mettaton:
Alphys is the one who forces him to be programmed to express confidence. She's in full control pf Mettaton's yellow gloves (With Mettaton having no control over them).

However, Mettaton's malfunctions is the only thing that has him out of this forced program.

Amalgamates:
Amalgamates are group of fused together monsters that couldn't withstand the dose of DETERMINATION injected into their SOULs by Alphys due to "Fallen down."
Alphys supposedly created them this way because "science requires sacrifice", despite the suffering the fused monsters endured together.
They suffered before after the "fall", but after DETERMINATION and the merger they arrive in agony and bitterness.

Undyne is a fish-like monster with long red hair, blue scales, a big yellow eye and an eye covered by a eyepatch, fish-like ears, red lips and sharp yellow teeth. She wears a black short shirt with a red outline of a heart on it, long red gloves, black pants with a wrestle mania belt and long red boots.

Undyne is almost as merciless as Asgore. She doesn't get angry easily and doesn't get in a fight immediately either. She never lets down once she's in a fight. She loves sparring with Papyrus, but doesn't admit it. She also loves showing off her own strength.

Flowey is shown to look almost the same as his Undertale counterpart. However, the petals are shown to be torn. His eyes are black. His face is white or peach and his petals are gold.

Flowey fears every monster, and even though he's been beat to the ground and nearing death he's always optimistic. Flowey isn't evil or cruel, instead he's kind, caring and cheerful.

Flowey was once known as Asriel, King Asgore and Toriel's son. After Chara died of being poisoned by Asgore, Asriel used Chara's soul to cross the barrier and become the God of Hyperdeath. He gave Chara their wish of dying peacefully in a bed of golden flowers. Unfortunately, the humans thought he had killed Chara. Asriel refused to fight and showed mercy. The humans didn't accept, and Asriel got brutally attacked and burned. Barely alive, he made it back to the Underground, where he died. His dust blew onto a bed of golden flowers. One flower was injected with determination, possibly by Alphys, Then he became Flowey. Toriel leaves Asgore because she finds out that Asgore was the one who poisoned Chara.

Mettaton is a main character in Underfell. He is a robot built by Alphys and the beloved star of the Underground.

Normal Form:
In his normal form, Mettaton appears as a curved metal box. He has two arms with his left hand donning a red glove and a yellow one for his right. He has black shoulder pads, three dials and the bottom of the body and a bow at the bottom of his wheel.

EX form:
Mettaton takes on a much more humanoid appearance. He has greasy black hair, two pairs of eyes, sharp hips, latex boots, a cracked red chest piece, and a mouth which is permanently locked in a menacing smile. His defining feature is his two sets of arms, his first pair wears red latex gloves while his second pair wears yellow lab gloves that are controlled by Alphys.

While Mettaton expresses confidence in his appearance, this is nothing more than a forced program by Alphys. In reality, Mettaton is very disappointed in how his body looks. He occasionally malfunctions which allows him to temporarily break free from the forced programming. He is hilariously overdramatic, often swooning and using the phrase "teehee". He also has zero subtlety when it comes to expressing disgust and will do so very plainly.