Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
Another theme is friendship or partnership. They often come together to face challenges in the Underground. For example, in many fanfictions, they join forces to defeat a powerful monster or solve a mystery that threatens their world. This theme of unity despite their differences is really appealing.
The contrast between the two characters. Underswap Sans is more laid - back and friendly compared to Underfell Sans who is more rough and edgy. When they interact in fanfictions, it creates a lot of interesting dynamics, like how they might clash or gradually understand each other.
The character development of Sans in this type of fanfiction is usually centered around his journey from a victim of abuse to a survivor. Initially, the abuse has left him with deep - seated insecurities and a negative view of the world. However, as he is exposed to positive influences, such as a caring friend or a new opportunity, his character begins to shift. He might start to develop new skills or interests that were suppressed due to the abuse. His relationships also change, from being guarded to more open and trusting. This development is often a slow and arduous process, but it is what makes the character so interesting in these fanfictions.
A plot could revolve around an accident that causes their universes to merge partially. Underfell Sans and Sans find themselves in a strange new reality where they have to figure out how to get back to their own universes. Along the way, they learn about the differences and similarities in their lives, and this leads to some really interesting character development.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including the Undertale AU crossovers like Underswap Sans x Underfell Sans. Another place could be Fanfiction.net, though the quality may vary more there. Additionally, Tumblr sometimes has users sharing or recommending such fanfictions in the Undertale fandom tags.
The fan base loves these characters so much that seeing them interact in new and unexpected ways is always a treat. The relationship between them can be explored in so many different ways, from friendship to rivalry. And the unique art styles and interpretations that often come with these fanfictions add to the overall appeal. There are also some really emotional stories that can be told about them finding common ground or learning from each other, which really tugs at the heartstrings of the readers.
You can find 'underfell sans fanfiction' on Archive of Our Own (AO3). It has a large collection of fanfictions from various genres, including Underfell Sans ones. Another place is Fanfiction.net. There are many users who post their works there.
Well, in 'underfell sans x papyrus fanfiction', their relationship is a key aspect. It can vary greatly depending on the author's interpretation. Some may focus on the loyalty between them, despite the cruel environment of Underfell. Others might explore how their personalities clash or complement each other. For example, Sans' cynicism in Underfell might contrast with Papyrus' more naive side, creating interesting storylines and character developments.
Sure. There are quite a few. In these fanfictions, the interactions between Underfell Sans and Frisk can be really interesting. For example, they might start off as distrustful of each other since the Underfell world is more hostile. But as the story progresses, they learn to rely on one another. It could be that Frisk's kindness starts to break through Sans' tough exterior in this version of the story.