Undertale: Multiplay is an Undertale AU/custom adventure animation (it is unclear what the author's stance on this is) made by HUGE in which two Frisks fall into the underground at the start of the game. It also involves changes to core gameplay mechanics and the behavior of some characters.
The Two Frisks
In the prologue, Frisk1 (according to the creator in a Q&A video, the two Frisks are Frisk1 and Frisk2) falls down first, but then soon finds another Frisk that has also fallen down where they did. From then on, they seem to be on a Neutral Route, as per the original game's requirements. Otherwise, the game's plot remains relatively the same.
Frisk1 has slightly shorter hair than the Frisk in the original game, while Frisk2 has slightly longer hair and a very small difference in face height. Their clothing seems to be identical to the original Frisk's. In battle, Frisk1 and Frisk2's names are Player1 and Player2, respectively. Both have red souls, are genderless/non-binary and address each other as Frisk. Both have a higher-pitched version of Toriel's sound bite, although Frisk1's sound bite is lower-pitched than Frisk2's.
They mutually care for each other because, according to them, they are the same person. Frisk1 seems to be more focused and serious about getting out of the Underground and protecting Frisk2, even to the point of doubting, ignoring or chastising other characters, including Frisk2 when they lie about what they said to Sans their opinion towards Papyrus's clothing. Frisk2 seems to be more eager to interact with and befriend other monsters along the way, but they also seem more childish and sensitive.
- All text is in Korean, although there is accurate (albeit confusing due to the way the text is displayed) English translation that can be enabled.
- Other characters are (obviously) aware of and sometimes surprised that there are two humans.
- The SAVE (and SAVE points), CONTINUE and RESET features are so far absent, and as seen in [2-A], there are no timelines or any kind of gaming-based ability, aside from LEAVE and GIVE UP, but it is unknown whether those options are part of the universe's game mechanics or even what their functions are.
- Realistic changes and elements are added, including: realistic injuries that affect the two Frisks' appearance and physical state in the overworld as well as their performance in-battle; the lack of timelines (due to the absence of the SAVE, CONTINUE, RESET functions), ability to move freely through the landscape (such as by hanging off a cliff)
- The two Frisks can talk, although they alternate between directly talking with their face sprite in the dialogue box and being narrated and having their speech indirectly shown. Before [2-1. Introduce], they didn't talk, and communicated in the overworld using exclamation marks above their heads (identical to ones that appear when you encounter a monster in the original game] or question marks, or even by simply looking at each other.
- Characters' emotions and status, especially the two Frisks', are portrayed in greater detail in the overworld.
- When Frisk1 (only Frisk1 shows this ability so far) is angry or serious, their text turns red and the sound bite changes. In more severe occasions, the dialogue box and text turns red and shakes.
- According to a Froggit, pressing "X" allows the two Frisks to hold hands instead of skipping dialogue, while pressing "V" allows them to interact with each other (in the overworld, so far)
- In battle, the two Frisks have their individual dialogue box and interface. A monster can attack them both at once or separately, based on how they use the options. In the former case, the bullet boxes of the two Frisks are merged. When either Frisk dies, their interface is shattered and falls down and disappears. The two Frisks can use the options on one another as well. When one Frisk talks with the other, the other's ACT button glows purple and when the other Frisk is selected, they turn to see and talk to them. Their actions (FIGHT, ACT) are completely separate from each other, except in one case where Frisk2 stops Frisk1 from FIGHTing Dogi by ACTing. The ACTs available do not stay the same throughout the battle. Also, the game mechanics can be broken, as seen when Frisk1 attacks multiple times in rapid succession. Monsters can even interrupt ACTs to attack.