The protagonist's SOUL returns to Green Mode, preventing the protagonist from fleeing. If Undyne's health reaches 0, she manages to cling on to life with Determination and survives further attacks from the protagonist. This is most likely to ensure that she always turns the SOUL red three times in the Neutral Route. If Undyne's HP reaches certain values while in Green Mode, she cuts the current Green Mode cycle short and turns the SOUL red ahead of when she would normally do it. At this point, the protagonist can either keep walking or pour water on her, the latter letting her leave and making the Undyne hangout available on the True Pacifist Route, or a Neutral Route where no monsters have been killed and Papyrus has been befriended. Then, Undyne slowly tries to reach them, but she collapses and faints due to the high heat, leaving her unable to attack. The protagonist must then run across a bridge until they reach an area with a water container. If the protagonist reaches Hotland, Undyne stops for a moment to look at Sans sleeping at a checkpoint allowing the protagonist to outrun her.
Continuing to flee into Waterfall causes Undyne to halt her pursuit, but she re-initiates her fight when the protagonist returns. The protagonist can also escape back the way they came from, although Undyne catches up quickly, as she enters the screen right behind the protagonist. If the protagonist does not flee, Undyne continues attacking, and the speed of her attacks continues to increase. Fleeing from her as soon as possible makes the battle significantly easier than trying to weather the three required rounds of Green Mode in one go, as she progressively increases her projectile speed over the course of the fight while fleeing resets the speed. If the protagonist does not escape while in Red Mode, Undyne throws more spears and then switches them back to Green Mode for the next round. Undyne the Undying, however, must be fought.
Due to her fast speed, she catches up with them and switches them back to Green Mode until they have endured at least three rounds, with each one being harder than the last. When the protagonist flees, she chases them down. Once in Red Mode, the protagonist can escape. After completing a number of green phases (first 6, then 4 and then 8), she switches the protagonist to Red Mode while throwing a spear that can be dodged by moving down. To spare Undyne, the protagonist must deflect her spears while in Green Mode and flee from her while in Red Mode (they cannot escape in Green Mode), gradually making their way through the cave she was guarding. These attacks are Genocide Route exclusive. She has another version of this attack in which the spears do not rotate, but instead head straight towards the SOUL the spears travel in the direction they were initially facing for a few seconds before fading away.