* Undyne is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursued the protagonist through the entirety of Waterfall before befriending them (with Papyrus' insistence help).
* Hot headed and passionate, she is the most determined monster of the Underground. SHE NEVER GIVES UP... Except if she is suffering from a heat stroke in the middle of Hotland.
* She conceded in training Papyrus to become a royal guard, even if, deep down, she wishes him to give up on his project. She can tell he is too innocent to fit the armor and secretly tries to reorient him by teaching him how to cook.
* Doing so, she bonded with the skeleton and now sees him as her best friend. She often sleeps at his place.
* She is very protective of all monsters and especially kids. She feels them all vibrating in her heart, which gives her strength and determination. This trait makes her also loved and admired by many.
* At first annoyed by Frisk's attitude, she learns to see their good sides and become their friend, despite not carrying a big torch for the human kind. She however admires their culture and giant weapons, through Alphys' readings, after being (wrongly) told by her those were real history, while being instead anime and manga.
* She was trained since childhood by King Asgore, after she failed in attacking him. Overtime, the mentor became her friend and the latest often comes to her house to share a cup of tea. However, sometimes before becoming the Head of the Royal Guard, she has done some community services, probably as a result of her hot temper and habits in beating up people, like the mailman.
* Undyne is secretive about her personal feelings, but with the support and openness of Papyrus, she decides to come out and confess her love to doctor Alphys, Asgore's royal scientist. Feelings that were happily and eagerly returned.
* She never properly met Toriel until everybody is reunited in front of the Great East Door, when Frisk finally breaks the barrier. From there, she learns to get along with her, but still sides with Asgore, no matter what.
* Despite being slender, Undyne is really, really strong. She doesn't approve of lies but respect people that show passions, even toward topics that she doesn't like. She is explosive in her habits and attitude, which makes her a very likable and exiting person to be around. She is missing an eye but doesn't seem to be very bother by it. She hates sweet and cold food, monologues, puzzles, laziness, lack of motivation, extreme cold, Hotland, and that her guests don't let her be a host and serve them properly, going up to throw spears at them if so. She is very, very uptight in making them feel "welcome"!
* Rather concerned in how people sees her but being oblivious of her popularity, she estimates that you shouldn't be liked because of your fame but because of your good doing. So despite being very straightforward and big mouthed, she is loyal and upright.