"I will not fall because of some god-forsaken HUMAN!" -Undyne, in the Genocide RouteUndyne is popularly interpreted one of the main antagonists of Underfell and Captain of the Royal Guard, although some interpretations have her as a subordinate to Papyrus, who has been promoted to captain. This varies, due to many people having Undyne be rebellious. Regardless of her position, she typically wishes to take Frisk's soul so she can please Asgore. She is also has a Genocide Counterpart, called "The Undying Demon". It activates during the Genocide Run.
Undyne's outfit is usually depicted as being similar to her Undertale counterpart's, save for having altered coloration and a more raggedy aesthetic. In these depictions her outfit is almost entirely black, save for red fin-edged gauntlets, occasionally red high-heeled boots, and a red drawing of a broken heart on her usually ripped shirt. Some depictions have any exposed skin on her arms and midsection wrapped in bandages. The fins on the sides of her head are also typically pointier than regular Undyne, and some artist illustrations give her darker skin and occasionally red freckles or spots on her face and body.
Her undying form is much different. It looks similar to her undying form in Undertale, but is more sharper and demonlike, hence her other nickname; "The Undying Demon".
Undyne is usually portrayed as deeply believing in Asgore's Words, and the general motto, "Kill or be Killed," and like Papyrus, shows no mercy to her foes. She is hotheaded and prone to fighting, only befriending Frisk if tricked into it, but otherwise not especially different from her canon Undertale counterpart.
Some popular fan interpretations, usually those that coincide with making Papyrus the captain of the Royal Guard, have Undyne act rebellious towards Asgore's rule or, on the opposite side, weaker and more unsure of herself in personality.
Frisk: Undyne attempts to kill Frisk at first, but starts to question herself when they show her mercy.
Alphys: Undyne is already dating Alphys in Underfell. While aggressive to almost everyone, she's very playful and assertive with her girlfriend, having no problems showing affection towards her in public.
Papyrus: Papyrus is Undyne's only real friend. While their relationship is tense, they get along better than most monsters in the Underground.
Underfell Game Differences Edit
In the fan game, Undyne is "a problem" for Papyrus, suggesting she is rebellious towards the Royal Guard, similar to above mentioned interpretations.