The Lost Souls are enemies fought during the battle against Asriel Dreemurr. They are the souls of the protagonist's friends who have to be saved to progress with the battle. Their fight is done in a boss rush fashion, where the player has to fight all the major characters in a similar way to their respective boss fights.
- The Lost Soul throws arrows at the protagonist from all sides in Green Mode and they must shield themself from incoming spears.
- Small rectangular Mettatons glide down on umbrellas while tossing hearts embroidered with the letter M as they reach the bottom of the screen.
- Bombs marked with a plus sign move downwards, which explode into lasers of a similar formation when shot with the yellow soul.
- Papyrus and Sans:
- The Lost Souls attack the protagonist with a series of bones scrolling across the bullet box, which can be dodged by jumping. Blue colored bones are used, but rarely.
- Toriel and Asgore:
- The Lost Souls move their hands along the sides of the screen and leave behind large fireballs that move towards the protagonist's soul after a moment.
- Partial circles of fireballs gradually spiral in from around the bullet box.
- To defeat a lost soul, one must ACT three times until their memories flood back.
- For Lost Souls that come in pairs, one must ACT four times in total. If all four ACTs are used on the same Lost Soul, then the other Lost Soul's memories will flood back immediately after the first one. Otherwise, both will regain their memories simultaneously.
- All humans will die! [Neutral]
- You're our real enemy. [Neutral]
- Mercy is for the weak. [Neutral]
- Well, some humans are OK, I guess! [After ACTing 3 times]
- You hate me, don't you..? [Neutral]
- I've got to keep lying... [Neutral]
- All I do is hurt people. [Neutral]
- No, that's not true! My friends like me! And I like you, too! [After ACTing 3 times]
- I MUST CAPTURE A HUMAN! [Neutral]
- THEN EVERYONE WILL. [Neutral]
- ... [Neutral]
- NO! WAIT!! YOU'RE MY FRIEND! I COULD NEVER CAPTURE YOU!! [After ACTing 4 times]
- just give up. i did. [Neutral]
- why even try? [Neutral]
- you'll never see 'em again. [Neutral]
- nah, i'm rootin for ya, kid. [After ACTing 4 times]
- This is for your own good. [Neutral]
- No one will leave again. [Neutral]
- ... [Neutral]
- Your fate is up to you now! [After ACTing 4 times]
- Forgive me for this. [Neutral]
- This is my duty. [Neutral]
- ... [Neutral]
- You are our future! [After ACTing 4 times]
- The Lost Soul(s) appeared. [Encounter]
- The Lost Soul(s) stand(s) there. [Neutral]
- Suddenly, [the/its/her/his] memories are flooding back! [After ACTing 3-4 times]
- You asked the Lost Soul to teach you how to cook. She doesn't know why, but she kind of wants to teach you how... [Recipe]
- You tapped the Lost Soul lightly. Something about the way you fight is familiar to her... [Fake hit]
- You gave the Lost Soul a big smile, like you remember she likes to do. For some reason, she sort of wants to smile back... [Smile]
- You clashed against the Lost Soul with all of your might. She feels your fighting spirit... It's familiar somehow. [Clash]
- You tell the Lost Soul that you'll continue to support her. Something about the way you said that is familiar to her. [Encourage]
- You call the Lost Soul on the phone... She starts to sweat. She doesn't know why, but this all seems very familiar... [Call]
- You ask the Lost Soul for help on a quiz question. She barely holds back from giving you the answer... [Quiz]
- You ask the Lost Soul what her favorite cartoon is. She can barely hold back from giving you an enormous answer... [Nerd Out]
- You told the Lost Soul a bad pun about skeletons. He seems to hate it... But the other Lost Soul seems to like it. [Joke]
- You asked the Lost Soul for help with a puzzle. He doesn't know why, but he really wants to help you. [Puzzle]
- You asked the Lost Soul to cook something for you. The Lost Soul is trying to hide its joy... [Recipe]
- You insulted the Lost Soul. Somehow, it seems flattered by this. [Insult]
- Seeing how nicely you treated its brother, the other Lost Soul remembers, too! [After ACTing 4 times, if all of them were towards one Lost Soul]
- You asked the Lost Soul to take a break from fighting with you... It seems like it wants to. [Take Break]
- You told the Lost Soul a bad pun about skeletons. He seems to love it... But the other Lost Soul seems to hate it. [Joke]
- You tell the Lost Soul you think jumble is tougher than crosswords. (It nods its head, like it knew this without question./It seems to have some dim recollection of this fact...) [Jumble/Crosswords; depends on choice at Sans's puzzle]
- You told the Lost Soul you'd think about what you'd done. It seems like it's trying to remember... [Judgment]
- Seeing how nicely you treated its brother, the other Lost Soul remembers, too! [After ACTing 4 times, if none of them were towards Papyrus]
- You tell the Lost Soul that you have to go if you're going to free everyone. Something is stirring within her... [Talk]
- You refuse to fight the Lost Soul. Something about this is so familiar to her... [Mercy]
- You tell the Lost Soul you prefer (Cinnamon/Butterscotch) instead of (Cinnamon/Butterscotch). Somehow, she faintly recalls hearing this before... [Preference; depends on choice at the beginning of the game]
- You hug the Lost Soul and tell her that you're going to see her again. Something about this is so familiar to her... [Hug]
- Seeing her remember you, the male Lost Soul tried hard to remember you, too! [After ACTing 4 times, if none of them were towards Asgore]
- You tell the Lost Soul that you're going to save everyone. Something stirs deep within him. [Talk]
- You stare deep into the eyes of the Lost Soul. He remembers the gaze of humans past... [Stare]
- You hug the Lost Soul. It seems like his aggression is slowly melting away. [Hug]
- You tell the Lost Soul that you won't hurt him, no matter what. Something about this is so familiar to him... [Mercy]
- Seeing how nice you are, the female Lost Soul's memories returned, too! [After ACTing 4 times, if none of them were towards Toriel]
- Unlike the other Lost Souls's dialogue, Sans's dialogue sways. This movement is not the same seen when characters are afraid; that motion is akin to trembling.
- Sans, unlike Papyrus, doesn't use the Comic Sans MS font or his unique sound bite before being saved.
- Attacking Undyne's SOUL will cause "MISS" to show up several times instead of once.
- Using the Butterscotch pie while fighting Asgore and Toriel will make them remember the smell, requiring the protagonist to ACT only one more time to help them recollect their memories.
- This is the only instance in the game where the protagonist battles Alphys.