Undyne Undertale first appears to the player when Frisk, the human child the player plays, leaves Snowdin and enters the next location called Waterfall. Here he has to avoid attacks from her spears, after which they will meet again later and then they have a battle.
In the game Undertale, she appears as a rather combative character with leadership qualities. Their manifestations can be seen when she is cooking with the main protagonist, and you also need to take into account the fact that she is the head of the Royal Guard. Their job is to find people who find themselves in the Underground.
If the player makes decisions that ultimately lead him to neutral endings. If Toriel becomes queen again, then Undyne leaves his post and becomes a teacher, and also helps the scientist Alphys with her research.
In situation, if the main protagonist nevertheless killed someone, did not become friends with her, then she is extremely upset that the main protagonist will turn out to be a murderer. She will lose everything that she has and is going to live on the sofa at the house of the skeleton brothers. And also she is going to live with the thought of how she wants to take revenge on the player for what he did.
In case Toriel is overthrown in one outcome of the events in the game, then the head of the Guard continues the work of Asgore with even greater zeal than the former king had.
If the main protagonist kills Toriel while passing the game, then the former head of the Guard will take the throne and ultimately militarize the Underground, as well as introduce new rules about people. Now every person who gets here will be killed.
In the good ending of Undertale, if Frisk hasn’t killed anyone in the whole game, then friends are going to come to the surface. Also, the player has a special mission in the story, during which he will need to deliver a letter for Alphys, after which Frisk and the scientist will begin a date.
Otherwise, if the player exterminated everyone he saw, then the head of the Guard will enter the state of Immortal Undyne, and Alphys is going to watch their battle, which in case of anything is going to evacuate all the monsters.