"   SCREW IT!
    WHY SHOULD I TELL THAT STORY...
    WHEN YOU'RE ABOUT TO DIE!?!
    NGAAAAAHHHHHHHHHHHHHHHHH!       "
    - Undyne before her battle

Undyne , known as StrongFish91 on the UnderNet, is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall and is frequently evaded or inadvertently thwarted by the Monster Kid. 

Undyne is passionate about everything that she does. She acts quickly on her ideas and is steadfast in her beliefs.
 Though she wants to defeat the protagonist, she also tries to be fair and offers an explanation of how Green Mode works. Asgore taught her the value of pacifism, which is why Undyne sees the protagonist as "a wimpy loser with a big heart." When the protagonist first encounters her, she fights to further Asgore's plan of collecting seven human SOULs but, if the protagonist shows mercy, Undyne just as quickly defends them from Asgore.

She loves to help others and mentors Shyren and Papyrus in various skills. Undyne dislikes puzzles, loves japes, and enjoys anime, an affinity she shares with her romantic interest, Alphys. Since Alphys told Undyne that anime is actually human history, Undyne regards human history as exciting and cool.

Out of all encountered monsters, Undyne possesses the most Determination. She melts before her death, implying that her body cannot handle the amount of Determination she has.