Well, there are definitely some out there. Some focus on their shared experiences in the Underground. For example, a story could be about how they support each other through various challenges. Another might be more about their post - Underground life and how they adapt to the surface world together. It really depends on what you're interested in.
There are quite a few. In some fanfictions, the relationship between female frisk and male chara is developed in a really unique way. It could involve their different personalities clashing at first and then gradually coming to understand and care for each other. There are also stories that might involve them on adventures together, facing new enemies or exploring new places. You just have to dig around a bit to find the really good ones.