About the AU
The basic idea of the AU is that characters are swapped in groups of 4, most of which consist of two Underswap pairs. The following groups are included:
- Sans -> Alphys -> Papyrus -> Undyne -> Sans
- Toriel -> Napstablook -> Asgore -> Mettaton -> Toriel
- Asriel -> Frisk -> Monster Kid -> Chara -> Asriel
- Flowey -> Annoying Dog -> Temmie -> Gaster -> Flowey
- Muffet -> Aaron -> Grillby -> Shyren -> Muffet
- Nice Cream Guy -> Snowdin Shopkeeper -> Burgerpants -> Gerson -> Nice Cream Guy
- Catty -> Bratty -> RG01 -> RG02 -> Catty
Besides characters, other things are changed as well. Similarly to Underswap, the seasonal themes of each main location are changed, but to the next season instead of the opposite season. More minor locations have been changed as well to fit the characters themselves.
New story elements are also added. The most notable one is the darkness, which is shown to turn monsters and humans who absorb it more evil, and, if it is absorbed too much, it can even kill them. It is also the material of which Gaster was created.
Not much is known about the battle mechanics at the moment, but they're known to be somewhat different due to the fact that the playable character is a monster, not a human. However, the concepts of LV, EXP, HP, ATK and DEF still exist.
- Icy Temple
- Blooky's Castle
- Asriel is now the protagonist of the game. Just like in Undertale, he is the son of Toriel and Asgore (though he doesn't know his father until during the game). Asriel's personality depends on the player's choices. He mostly appears in his child form, but if he absorbs Frisk's SOUL, he turns into his God of Hyperdeath form. His name is only revealed in the end of the game.
- Gaster is the first character the protagonist encounters in the game. He appears to be a friendly skeleton at first, but when he shoots the protagonist with Gaster Blasters ("friendliness beams"), his true nature is revealed. In a neutral route, he turns into Interdimensional Gaster after the battle against Napstablook. He is in fact Chara after being resurrected with pure darkness.
- Metta (formely known as Happstablook) is the owner of a mansion in the Icy Temple. He is popular in that area and hosts a TV show every day. He also acts as Asriel's guide in the Icy Temple.
- Calm Riverperson is the first monster Asriel encounters.
- Toriel is Asriel's mother. She is also close friends with Metta and works as a cook in his mansion.
- Undyne is the first character the protagonist meets in Meltin. She is calmer than her Undertale counterpart, but still doesn't appear to trust the protagonist. In the Genocide Route, she turns into Undyne the Undying and is the final boss of that route.
- Alphys is Undyne's wife and they live together in a house in Meltin. She works as a trap builder for Papyrus. Like her Undertale counterpart, she is nervous and likes anime.
- The dog members of the Royal Guard members haven't been swapped with anyone. Instead, they are changed into flower versions.
- Cactus is the first plant member of the Royal Guard that Asriel encounters. He is the most tsundere of the plants.
- The Echo Flowers are a married flower couple that Asriel encounters.
- Lesser Flower is a flower that grows every time Asriel waters it.
- Greater Flower is a playful flower.
- Gerson sells ancient items around the location. He is first seen in Meltin and later in Lavafall and Coolwood.
- Aaron is the owner of the gym in Meltin. Undyne trains there regularly along with the flower members of the Royal Guard.
- Nice Cream Guy is a rabbit who sells food, including his famous nice cream, in a shop in Meltin.
- Frisk is a determined human. They first appear in Meltin, where they quickly befriend Asriel. Frisk has a red human SOUL. They walk with Asriel in Lavafall and tell how cool Papyrus is. In the Genocide Route, Asriel can kill Frisk and steal their SOUL. In the other routes, Frisk stays as Asriel's friend.
- If the FUN value is correct, Asriel might meet hooman.
- Papyrus is the leader of the Royal Guard, a title he is very proud of. Like his Undertale counterpart, he loves puzzles and doesn't actually want to hurt the protagonist. He sees good in Asriel in almost every route. It is only in the Genocide Route when, after Asriel successfully kills Frisk, he stops believing in him.
- Dummy is the monster who takes Asriel into different places.
- Angry Riverperson is the cousin of Calm Riverperson, who, like Mad Dummy in Undertale, rages at the player. In the Genocide route, all of the raging causes them to fuse with their cloak, turning them into Happy Riverperson.
- Muffet is a normal monster that can be encountered. She loves money.
- Grillby is a fire monster who lives in Lavafall. He has a bird with him. He had a brother who was turned into an Amalgamate.
- Annoying Flower (Flowey) is a character who appears in various places. When he is encountered in Waterfall, he fills the inventory with flower seeds.
- Toby is a white dog species. They live in Toby Village, located in Lavafall. One of them is a shop keeper whose greatest desire is to create a successful video game (something you can fund).
- Burgerpants is a shopkeeper in Lavafall. He thinks that the Royal Guard is doomed.
- Sans is the Royal Scientist and the brother of Papyrus. Like Undertale Sans, he loves puns, and often posts puns online. He also created a new robotic outfit for Asgore in order to hide the fact that he is a boss monster. He is also the one behind the experiment that created Gaster.
- Asgore Dreemurr is Toriel's ex-husband and Asriel's father. When Asriel was very young, he and Toriel got into a disagreement, since Asgore wanted to explore the other side of the fence, while Toriel wanted to stay. In the game, he appears as Dreemurrbot, a robot created by Sans. In the Neutral and Pacifist Routes, he turns into True Dreemurr, a form in which he resembles a boss monster more than in his previous form. If Asriel spares him, he reveals than he is Asriel's father and his robot form is just a disguise. In the Genocide Route, he appears as Revenge Dreemurr, a form resembling Asriel's final form from Undertale.
- Shyren is a siren-like monster who wants to sell her albums. The problem, however, is that the albums are very expensive. Her albums are sold in Icy Temple has well, where they're a lot cheaper. She has a lot of agents.
- Bratty & 01 are members of the Royal Guard.
- Catty & 02 sell trash near the Dreemurr Resort. Both of them have close friends in the Royal Guard but they're not interested.
- Dreemurr Resort Shopkeeper is a bunny who sells food in the Dreemurr Resort.
- Napstablook is the king of the monsters. He has the same personality as Undertale Napstablook. He seems to miss his cousin a lot. In his boss battle, he absorbs darkness, turning him more evil.
- Chara was Napstablook's adoptive child. Like Frisk, they were a determined human. Unlike Asriel, who refused to kill humans, Chara wanted to kill them all, but was eventually stopped. In the end of the Pacifist route, Chara becomes good and regrets all the stuff they did in the past. They tell Asriel that he is the type of friend Monster Kid should have had.
- Monster Kid (no true name known) was the first known boss monster to cross Fence E-Wolf. They were adopted by Napstablook and was best friends with Chara. In the end of the Genocide route, Monster Kid appears as Goner Kid. Their name is whatever the player names the fallen boss monster.
- Temmie C. was the Royal Scientist before Sans. She wrote "entreeh Numba sevn TEEHN", which is actually about college instead of an experiment. Temmie's followers can be encountered during the game if the FUN value is correct.