Undyne's house appears to be a massive angry fish with the door resembling the mouth and the windows resembling the eyes. After completing the events that occur in this area, the house turns entirely black, and fire can be seen from the house's windows and slightly open door. The "eyes" also change from angry to teary.
The interior of the house consists of a kitchen with a table, a chair, a fridge that keeps food warm, a sword, a piano, and a window on the left side of the house. The only other door in this room that is not the entrance is the locked door that leads to Undyne's room.
Undyne's house will not yield anything of importance until Papyrus calls the protagonist in front of the "Welcome to Hotland" sign while being pursued by Undyne to say that he, Undyne, and the protagonist should all hang out sometime and then says to meet at her house later. Returning to this location after the call will cause Papyrus to show up in front of her house.
Befriending Undyne can be completed on a Neutral Route, but it is an integral part of the True Pacifist Route. The requirements for this event is that the protagonist must not kill anyone before starting this event or else Undyne will refuse to open the door and become friends with a murderer.
Once the protagonist approaches and initiates the hangout, Papyrus will knock on the door. Undyne answers the door asking if Papyrus is prepared for his special private training, he will then move saying that he has brought a friend. Undyne will have an angry look on her face before inviting the protagonist inside.
Papyrus will then jump out Undyne's window claiming that he has to go to the bathroom. Regardless of what the protagonist chooses, Undyne insists on becoming best friends with them. When the protagonist sits down at her table, she will ask what they would like to drink. Undyne will then throw a spear at the protagonist once they move and say that they should point to what they will like.
During the hangout with Undyne, she asks the protagonist to choose a drink:
- Pointing the spear at Undyne while she demands them to choose a drink will cause her to ask if the protagonist is flirting with her. Selecting her again will only result in repeated question marks.
- Selecting the Sword when she demands them to choose a drink will cause her to comment that she would gladly fill the protagonist with swords if they were not her house guest.
- Selecting the Fridge with the spear when she demands them to choose a drink will cause her to comment that she would never give the protagonist her entire fridge.
- Selecting the Hot Chocolate when she demands them to choose a drink results in her telling the protagonist that the Hot Chocolate box is empty; she stopped getting it because Asgore kept getting marshmallows stuck in his beard.
- Selecting the Soda when she demands them to get a drink results in her about to get it for the protagonist, but then realizes that they do not look like they want it. She goes on to say how soda "rots your fighting spirit," then as if the protagonist asks her, she says: "Why do I have it?" and then makes an expression similar to (:3).
- Selecting the sugar when she demands them to get a drink will cause Undyne to tell them that the sugar is for the tea, and that she's not going to give them a cup of sugar. She then asks if she looks like the ice-cream woman, following up by asking if human ice-cream women "terrorize humanity with energy spears" and if their ice-cream songs are a "prelude to destruction." The player is then given a yes/no prompt: if the player chooses "no," she replies with "That's what I thought." Choosing "yes" will cause her to reply with "... what? REALLY? That rules!!!"
- The tea, literally called the "blatantly correct choice," is the only option that allows you to get a drink.
Upon choosing the tea box (other answers only lead to another choice), she will then make tea, then sit down at her table. She will then talk about Asgore and Papyrus, and explain how she came to lead the Royal Guard.
She will then remark about how Papyrus missed his cooking lesson and demands that the protagonist takes it in his place. The attempt to make spaghetti will go horribly wrong, causing the house to burst into flames. Undyne will challenge the protagonist to a "final rematch" to regain her lost pride. After she realizes that the protagonist does not want to harm her, she will stop fighting.
Undyne will then run off to see Papyrus as her home is now on fire. She will then be seen in front of Papyrus and Sans's House. It the protagonist speaks with her there, she will give the protagonist Undyne's Letter if their inventory is not full and will say to deliver it to Alphys.
- When Papyrus stands in front of Undyne's house before the protagonist befriends her, She's Playing Piano begins playing.
- The Mad Dummy appears outside Undyne's house after it is fought.
- Undyne is neighbors with Napstablook. When called outside of their house, she asks the protagonist to be kind to them.
- Knocking on the door of the house at certain points in the game can result in varied dialogue:
- Knocking on the door after fighting Undyne but not giving her water: "You hear irritated groans from the inside."
- Knocking on the door after fighting Undyne and giving her water but before hanging out with Papyrus: "It doesn't seem like anyone is home."
- Knocking on the door after killing Undyne: "It doesn't seem like anyone is home. And you get the feeling no one will be."
- Knocking on the door while the house is burning after befriending Undyne: "Sounds like only fire lives here now."
- Knocking on the door if Papyrus is in front of it after fighting Undyne on the Neutral Route: "If someone's there, they aren't answering the door."
- Papyrus's small animation where he dances or walks in place before he enters Undyne's house is supposed to represent him wiping his feet on the mat in front of the house.
- Calling Papyrus in this area immediately after the hangout with Undyne will result in special dialogue.
- ↑ "I like when Papyrus weirdly wipes his feet before going into Undyne's house." - Toby Fox. January 11, 2016. Twitter.
- ↑ Jacksepticeye triggering the special dialogue