Undyne

Undyne is a character from the game Undertale. Initially appearing as the antagonist of the Waterfall section of the game, the player can either befriend her in the pacifist or neutral route, or face her in a particularly grueling challenge on the genocide route. While not as popular as, say, Sans, her battle in the Genocide route and memorable character shot her up to being one of the more notable sexywoman out there.

Basic Information
Undyne is introduced as a stoic figure in armor seen near the entrance to Waterfall, where she either hears Papyrus' report or doesn't depending on whether you spared him or turned him to dust. From there, she relentlessly antagonizes you throughout the Waterfall section, all the while maintaining her image as a cool, collected warrior...right up until you actually fight her, at which point she ditches her helmet and reveals her true, more boisterous personality and fights the player. In the genocide route, she serves a bit of a different purpose - after saving Monster Kid from a devastating blow, she nearly dies herself before her inner determination kicks in and lets her reach a new, stronger form, providing a much bigger challenge than a good amount of bosses in the game.

For more information, see her page on the Undertale wiki.

Anthro
Fish lady.

Technically Antagonist
Ask any Undertale fan whether they consider Undyne a villain, and they'll probably look at you funny for even suggesting that. However, she does pursue and hunt down the player without mercy, and unlike the previous area boss, Papyrus, is fully aware of her duty as a guard and is very willing to kill you. Heck, she's one of the few bosses who can't be reasoned with; if you want to spare her, your only option is to just get out of dodge as soon as you get the opportunity and outlast her in Hotland.

Power
Her genocide boss fight as Undyne the Undying is one of the hardest in the game, bar none, and as such, she's noted to be rather powerful in several AUs and fanworks.

Girlboss
Most characters wouldn't get away with attempted child murder, especially if that child was also the player, but Undyne gets away with it because of her fun personality and heroic actions in the genocide route.

Dominating
Very bold, brash, and aggressive, both in pursuing the player and in her relationships with others, as can be seen in the phonecalls she has with Papyrus.

Eye Imagery
Has one eye missing and covered with an eyepatch, which gains a glowing light as Undyne the Undying. The fact that she has both eyes in Deltarune is a sign to the player that something's off.

WLW
Canonically gets together with Alphys at the end of the pacifist route.

Theme Song
Spear of Justice, NGAHHH!!, and Battle Against a True Hero.

One-Winged Angel
Undyne the Undying.

Quotable Catchphrase(s)
"ANIME IS REAL!" and the entirety of the speech she gives in the Genocide Route right after she gets Monster Kid to flee.

Marked Canon/Fanon Divergence
This has less to do with her actual personality (though that is a thing to some degree) and more with her appearance, as her battle sprites and overworld sprites...don't really do her appearance much good. As such, a lot of fanworks tend to draw on her appearance when she dressed up for a date with Alphys, with more hair, a spruced-up look, and a generally more conventionally attractive appearance overall.

(Vriska)ization
While not having as many as Sans, Undyne does have her own AU counterparts from the various popular Undertale AUs.