UnderKin is an AU giving most characters additional friends/Family Members, such as Bree Serif of the Skeleton Family, changing the plotline drastically.
About the AU
In UnderKin, many characters have extra friends or family such as Sans and Papyrus' new little sister, Bree Serif. Even Frisk has a twin named Frosch. The ability to do genocide or mercy relies on Frisk's survival.
In this AU, the other children didn't die, but instead willingly gave DNA samples to help with the gate, save for Chara but that is another story. Asgore still needs at least one human soul to finish the project, which causes him to be at odds with Toriel after their son disappears.
We start with the player characters, Frisk and Frosch, falling down the hole together. Toriel saves the twins from Flowey. Frosch, who is normally skeptical and doesn't like monsters, actually takes a liking to Toriel and wants to stay at her house forever. He tries, and fails, to convince Frisk to stay, and ends up going with her because he wants to make sure she is okay.
Shortly after leaving the Ruins, the twins are approached by Asgore, who tells them about the project, and very politely asks if one of them would donate their soul to the cause. This is where the road splits.
In the Mercy Route, Frosch follows Frisk's example and is nice to the monsters. Frisk and Frosch work together to convince Asgore to focus on what is important to him--his family. Asgore confesses that he misses Toriel, and agrees to drop the project in favor of finding his son and patching things with his wife. He thanks the player and leaves. This means that Asgore is no longer a final boss, and instead, there is only Flowey. When you mercy Flowey, you use what power the project did have to revert him to his regular body. He is incredibly grateful and returns home to his family. The next time you visit him, he proudly introduces you to his new baby sister.
- Alternate Mercy Route: If Frosch dies instead of Frisk in the fight with Asgore as a result of becoming violent, Frisk will do the mercy route like normal, carrying her brother's soul all the way to the gate and releasing the monsters into the overground. She is rewarded for this deed by the DNA of the other children coming together with Frosch's soul to recreate him and bring him back to life. He looks different but is the same brother.
In the Genocide Route, Frosch decides to attack Asgore because he feels his and his sister's lives are in danger. This causes Asgore to retaliate until one twin is dead. If Frisk dies, he disappears with her soul, leaving Frosch with a choice: Reset or Kill them all. If he chooses to kill them all, he goes about killing each and every monster he finds until he has gotten through Asgore and Flowey and then uses the power of the gate to destroy the entire monster world.
- Alternate Genocide route: If Frosch chooses to reset, the game restarts with no changes. If the player has him make the same choices that would result in Frisk's death again, Frisk will stop his hand upon the first attack and you will see a screen of Frisk looking at him sorrowfully, saying "Didn't you learn anything from the first time?". The screen then flickers and blood pours from her eyes, and the game resets itself again like normal. She will change the number she says in her phrase to match how many time the player has tried to cause Frisk's death.
- Alternate Genocide Route #2: When the player faces the SkeleFam, the order in which they kill them matters. If he kills Serif first, there is no going back--the brothers will fight the PC to the death. If he kills either of the brothers first, the other brother and Serif will face the PC. Mercy becomes possible during these fights, but only once Serif's wailing summons innumerable Gaster Blasters. If you kill both brothers, Serif will face you, catatonic, and there is no way to defeat her Gaster blasters without resetting and trying another combination. Mercy in any of these situations results in reset after a short scene where the player realizes they have done to this family what was done to him, and how his sister would hate him now.
As is the point of the AU, there are several additional characters in relation to the original cast. The original creator is still working on these and may add more in the future.
- Sans/Papyrus: Bree Serif (Little sister)
- Toriel & Family: Asteria (New baby/Asriel's new sister)
- Alphys: Brys (Lab Partner)
- Undyne: Baron (Brother)
- Monster Kid: Monster Girl (Sister)
- Mettaton: Mira (Prototype/Lovely Assistant)
- Grillby/Fuku: Mai (Wife/Mother to Fuku)