Twilight Sparkle from MLP is often a popular choice. Her intelligence and powerful magic make her a great character to interact with the complex world of Undertale. In Undertale characters, Sans is also quite popular in these crossovers. His laid - back but mysterious nature can create interesting dynamics when paired with the ponies.
Well, in 'mlp meets undertale fanfiction', there could be crossovers of characters. Picture Fluttershy befriending a shy monster in Undertale. Her kind nature could help the monster come out of its shell. Also, the magic system in MLP could blend with the combat or determination - based mechanics in Undertale. It might create new ways for the characters to overcome challenges. For instance, Twilight Sparkle's magic could be used to solve some of the puzzles in the Underground in a different way than the normal Undertale characters would.
One idea could be that the ponies from MLP end up in the Underground world of Undertale. Maybe Twilight Sparkle could use her magic to interact with the magical elements in Undertale, like the SOULs. Another idea is to have Sans and Papyrus meet Fluttershy. Papyrus would be all excited to show off his 'coolness' while Fluttershy might be a bit shy at first but then they could become friends over their love for cooking (Papyrus loves making spaghetti).
It's a fan - made concept where the worlds or characters of Underswap and Undertale interact. It often features unique storylines that combine the different character traits and settings from both.
Well, it could be really interesting. Maybe it shows a unique interaction between Chara from Storyshift and Sans from Undertale. Their personalities are quite different. Chara can be rather complex and Sans is known for his laid - back yet perceptive nature. So in the fanfiction, there might be a lot of unexpected exchanges and development of their relationship.
The characters could interact through a portal that connects their two worlds. For instance, Applejack could enter the Undertale world and meet Toriel. Applejack's honesty and hard - working nature might impress Toriel, and they could exchange stories about taking care of others (Applejack on the farm and Toriel with the lost children).
The merging of the two universes' concepts. In RWBY, we have the concept of Grimm and Huntsmen, while in Undertale, there are monsters and the whole system of LOVE and EXP. When these are combined in fanfiction, it can lead to really unique storylines. For example, how would the RWBY characters react to the moral implications of the Undertale world's combat system? It makes for some deep and thought - provoking stories.
Chara is often a popular character. Their complex nature makes them interesting to explore in the context of Storyshift. They can be portrayed in different ways, sometimes more sympathetic than in the original Undertale.
Sans is often a very popular character in these fanfictions. In both Undertale and Underfell, he has a distinct personality. In Underfell, his more edgy and somewhat darker side can create interesting interactions when combined with the regular Undertale Sans.
In such a fanfiction, there could be a lot of conflict. Underfell Sans is from a more hostile AU than Undertale. He might initially be distrustful when meeting the characters from Undertale. For example, he could misunderstand their kindness as weakness or a trick.