For the track with the same name, see Undyne
Undyne is a blue, anthropomorphic fish monster of similar height to Papyrus. She has scales, round glasses, and normally wears a white lab coat. However, she wears a black and white polka dot dress on her date with the protagonist. She has short red hair that is in a bun.
Undyne is a nerdy, shy recluse with a fondness for human anime and manga. She has crush on Alphys, but struggles to communicate her feelings.
Undyne is filled with guilt and insecurities about herself and her accomplishments. After she accidentally created the Amalgamates while experimenting with Determination, she told nobody and became depressed
Undyne supports the protagonist as they adventure through the Underground.
Undyne and Alphys both have crushes on each other.
Toriel asked people to get "fallen" monsters for Undyne's research on Determination.
Undyne will help you (and/or accidentally trick you) through the puzzles like Undertale Alphys does, and will talk about various anime shows that have entered the Underground. She will also hold the Amalgamates Underground in the False Tab until Underswap Chara goes in and does what Frisk does in the Pacifist run in Undertale.
In the genocide run, Undyne will rescue and evacuate as many monsters as possible, but in the end, you will just kill them all anyway. She will give you no interference out of fear that you may find her. You will be able to go to the core and fight the last boss before the Papyrus fight. It is unknown what happens to her after the Genocide run is complete although from evidence from the neutral points towards the fact that she may have committed suicide due to the horror of her situation.
Undyne initially meets the protagonist in her lab. She explains that she has watched the protagonist throughout their journey using hidden cameras. She helps the human through the rest of their journey until Napstaton admits that Undyne wanted to insert herself into the human's journey. You cannot enter the false lab during Neutral and Genocide routes.