The relationship could start with suspicion. Frisk, coming from their own world, might not trust this new Chara right away. But as they spend time together, they could find common ground. For instance, if they both have a love for the unique monsters in their worlds. This could gradually lead to a friendship where they rely on each other.
Well, Frisk could be cautious at first. Since Chara in Underswap is different, Frisk might be on guard. However, as they get to know each other, they could become friends. For example, they might share stories about their friends in their respective worlds. Frisk could talk about Sans and Papyrus, while Underswap Chara could talk about their own versions of the characters. Eventually, they could team up to face some challenges in the Underswap world.
It often starts with curiosity. They meet in the Underground and are intrigued by each other. As they spend more time together, they start to trust one another. For example, Frisk might show Chara the power of kindness, and Chara might show Frisk some of the secrets of the Underground.
Another theme is self - discovery. As they interact, both Chara and Frisk might learn more about themselves. For example, Chara could discover a more positive side of themselves through Frisk's influence. In some fanfics, this is a central aspect of the story, with detailed descriptions of their inner thoughts and changes.
One great 'Underswap Sans x Chara' fanfiction is 'A Different Bond'. It delves deep into their relationship, exploring how their personalities interact in a unique UnderSwap universe. The story has a great balance of action and character development.
There are fanfics that focus on the more emotional side of their relationship. For example, a fanfic might explore how Underswap Sans helps Frisk deal with the trauma of their previous Underground experiences. He could use his unique charm and understanding to comfort Frisk, which makes for a really heart - warming read.
Another theme is discovery. Since Frisk is new to the Underswap world, they discover new things about it with Sans. Sans, in turn, discovers new aspects of himself through his interactions with Frisk. This mutual discovery is a big part of many stories.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search specifically for ones involving Female Frisk and Female Chara. Another option is FanFiction.net, which also has a wide range of works. Just use their search functions and type in the relevant characters to find some interesting reads.
Another common theme could be the distortion of relationships. The relationship between Chara and Frisk is complex in the original game. In these fanfics, when Chara kills Frisk, it distorts that relationship completely. It could also lead to themes of guilt and regret, not just for Chara if the story delves into their emotions, but also for other characters who may have failed to prevent the killing.
This kind of 'fanfic' involves extremely unethical and disturbing content. Rape is a serious crime and violation of human rights in real life, and depicting it in fan - created works, even in a fictional context, is unacceptable as it promotes a negative and harmful view.