- Undyne is ready to throw her spear at the protagonist
Undyne is an anthropomorphic, piscine-like monster. Blue scales are visible on her head and she has a long, red ponytail. Red and blue fins are on her sides. She also has sharp, yellow-colored, protruding, teeth. Red eyeshadow is worn by her, and she has an eyepatch in her left eye. A black tank top and jeans are her usual attire, but she debuts in armor in both light and dark gray shades. Her eyes are black with yellow sclera and vertically positioned black pupils. Und yne doesn’t have a nose.
Undyne wears a white turtleneck underneath a black leather jacket, a scarf round her neck, pants and lipstick during the Alphys date. She also has a light-colored eye patch and some hair down.
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After her HP drops to zero, she can persist through Determination and her pupils will turn white. Her sclera also turns black if she persists with her determination. Her eyepatch is gone and her empty eye socket glows like a spear. Her hair becomes more pointy and rigid at the ends, and her fins rays appear to be shaped like a trident. Her new armor appears to be darker in color with souls/hearts on the chestplate, and she has new white gloves with spikes that protrude from her backside. She is also wearing boots with pointed toes.
Und yne is passionate in all she does. She is quick to act on her ideas, and steadfast in her convictions.  Although she is determined to defeat the protagonist, Undyne also tries fair and explains how Green Mode works. Asgore taught Undyne the importance of pacifism.  This is why Undyne regards the protagonist as a “wimpy loser with big hearts.” The protagonist meets her for the first time and fights to help Asgore collect seven human SOULs. However, Undyne will defend them from Asgore if she shows mercy.
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She is a great mentor to Shyren, Papyrus and other people in different skills. Und yne is not a fan of puzzles and loves japes  but she does enjoy anime, a passion she shares with Alphys, her romantic interest. Und yne considers human history exciting and cool, after Alphys explained to Und yne that anime was actually human history. 
Und yne is the most determined of all the monsters encountered. Her body is unable to handle the Determination she has, and she melts before she dies.
- Main Story
- Neutral Route
- When Undyne is close, the lighting changes dramatically
After entering Waterfall, the protagonist encounters Undyne. Undyne is perched on top of a ledge. Papyrus approaches Und yne if she is alive and gives a report about the human’s condition. Then he attempts to persuade her to stop harming them. The protagonist can go on, but Und yne is alerted by the grass rustling when Papyrus has left. While Und yne is playing, she looks at the grass briefly before retreating.
Und yne is a follower of the protagonists through Waterfall, and attacks them repeatedly. However, the protagonist manages to avoid capture by either avoiding her attacks or because of Monster Kid’s interference. Und yne later approaches the protagonist, while Monster Kid hangs from a bridge. The protagonist can save the Monster Kid from Undyne if they do so. Undyne will save the Monster Kid if the protagonist fails to help them. Und yne suffers a little bit of damage for this. This is helpful in killing Und yne along the Neutral Route.
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