Start by making a list of their distinct characteristics. Underfell Sans has a more angular appearance and a darker color palette, which can be reflected in his personality as being more cynical. Sans, on the other hand, is more approachable in appearance and personality. When writing their interactions, use these traits. Let their body language speak volumes. Underfell Sans might cross his arms and glare, while Sans slouches and gives a lazy grin. As the story progresses, these characteristics can be tweaked based on the events they experience. If they face a great loss together, it could make Underfell Sans more vulnerable and Sans more serious.
First, understand the character of Underfell Sans. He's different from the normal Sans, more edgy and with a darker personality. Then, create an interesting scenario for the reader to interact with him. For example, a dangerous mission in the Underfell world.
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.
There's a really cool fanfic called 'FellSwap Encounters'. It delves deep into the relationship between underfell sans and underswap sans. It starts with a chance meeting and then shows how they gradually understand and support each other. The author does a great job of showing their different characteristics and how they complement one another.
There's also 'Sibling Rivalry Reimagined' which delves deep into how their personalities from their respective universes interact. It has some really interesting character development for both Sanses. It shows how their differences can lead to both conflict and unexpected understanding. The author does a great job of portraying their emotions and the way they communicate, which is quite different from the original game's interactions.
To write a great Undertale Underfell Sans x Fell Toriel fanfiction, you need to research the Underfell universe thoroughly. Know the rules, the power dynamics, and the overall atmosphere. When it comes to their relationship, start with small moments. Maybe Fell Sans brings Fell Toriel a small gift, like a flower from the dangerous parts of the Underground. This simple act can start to break the ice between them. As for the writing style, use vivid descriptions to bring the Underfell setting to life. Describe the dark corridors and the menacing monsters in the background as their relationship unfolds.
There's 'Sans - Bound'. This fanfiction focuses on the challenges they face when they are forced to work together. It's not just about their relationship but also about the world - building in the undertale and underfell universes. The descriptions of the settings are vivid and add to the overall charm of the story.
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.
You can start by looking on fanfic platforms like Archive of Our Own (AO3). It has a large collection of Undertale - related fanfics, including those about Underswap Sans and Underfell Sans. Just use the search function and type in the relevant tags.
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.