A common theme is adventure. Since Frisk's story in Undertale is full of adventures, these fanfictions often continue that trend with the male reader character joining in. Another theme is friendship - turned - love. The relationship between Frisk and the male reader usually starts as a friendly encounter in the Underground and gradually develops into something more romantic.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfics, and you can simply search for 'female frisk x male reader' in the search bar. Another option is Fanfiction.net, though the tagging system might be a bit different. Some Tumblr blogs also reblog and recommend such fanfictions.
A common theme is adventure. Since Frisk is often associated with an adventure - filled journey in the original game, this often gets carried over into the fanfiction. The male reader might join Frisk on his quest through the Underground.
One place to start looking for male frisk x male reader fanfiction is Archive of Our Own (AO3). It has a large collection of fanfics in various genres and themes. You can search for specific tags like 'Male Frisk', 'Male Reader', and any other elements you might be interested in, such as 'Adventure' or 'Romance' to narrow down your results.
Love, of course, is a major theme. The love that develops between male frisk and female chara can be portrayed in different ways. It could be a slow - burning love, starting from a simple attraction and evolving into something deeper. Or it could be a more sudden, passionate love. These different portrayals of love add a lot of variety to the fanfictions in this genre.
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.
One common theme is redemption. Chara might be portrayed as having a troubled past, and Frisk helps him find a better path. Another theme could be growth, as they both grow as characters through their relationship. Also, adventure is a theme, with them going on quests together.
One possible plotline could be a love triangle where the reader is caught between Frisk's kindness and Chara's more complex nature. Maybe the reader meets Frisk first, and they have a sweet, innocent relationship. But then Chara enters the picture, bringing a sense of mystery and a different kind of charm. The story could revolve around the reader's internal struggle to choose between them, with different adventures and emotional moments along the way.
One common theme is friendship evolving into love. In many of these fanfictions, male frisk and female chara start as friends who have shared experiences in the Underground, and over time, their relationship deepens into something more romantic.
Redemption is also a theme. Asriel has his own past to come to terms with, and Frisk can play a role in that. Their relationship can be a catalyst for his redemption journey. For instance, in some fanfictions, Frisk's kindness and belief in Asriel help him move forward from his past mistakes.