Adventure is also popular. Undyne is a brave character, so fanfictions often have her and the reader going on all sorts of exciting adventures, exploring new places or fighting against threats.
One common theme is friendship. Since the reader is often involved, it shows how they build a friendship with Undyne, like learning to understand her warrior - like nature.
Self - discovery can be a theme too. The reader may start off a bit timid but through their interactions with Undyne, they find their own courage and strength, just like Undyne helps them realize their potential.
Overcoming challenges is also a big theme. Maybe there are challenges in their relationship due to differences in their nature (Undyne being a tough warrior and the reader perhaps being more timid). Or they might face external challenges like threats from other characters in the Undertale world, and they have to work together to overcome them and in the process, their relationship deepens.
Common themes include self - discovery. The male reader may start off not knowing much about himself or his place in the Underground, and through his relationship with Undyne, he learns more about who he is. Also, loyalty is a big theme. Undyne is a very loyal character, and the male reader has to prove his loyalty to her in many fictions. Additionally, there's the theme of protection. Undyne is a protector in the Underground, and in the fanfictions, she often tries to protect the male reader, and he may also try to protect her in return.
Friendship turning into love is also a very common theme. At first, the reader and Undyne might start as friends, but as the story progresses, their relationship deepens into something more romantic. And of course, the theme of loyalty. Undyne is a very loyal character, and this is often emphasized in the fanfics as she stands by the reader through various situations.
One common theme is the fish - out - of - water situation. Since Undyne is from her own game world, when she is placed in another universe in a crossover, she has to adapt. For example, in crossovers with modern - day Earth settings, she has to understand new technologies and social norms. Another theme is power comparison. When she meets powerful characters from other franchises, like in a crossover with the Avengers, there's a comparison of her strength with the likes of Thor or Hulk.
One common theme could be the challenges of pregnancy. Since Undyne is a strong character, it might be interesting to see how she copes with the physical and emotional changes that come with pregnancy. Another theme could be the relationship between Undyne and her partner during this time, like how they support each other.
She is also very determined. This trait can make for an interesting dynamic in the fanfiction. If the reader is facing a difficult task, Undyne's determination can inspire them. She won't give up easily, and that attitude can rub off on the reader and help them grow as a character.
Growth is also a common theme. Frisk may grow in confidence and strength under Undyne's influence, while Undyne might become more understanding and less hot - headed because of Frisk. And of course, there's the theme of adventure. Since they are in the Underground world which is full of mysteries, many fanfictions have them going on adventures together, facing monsters and solving puzzles.
The theme of growth is often seen. Both characters grow as individuals within the relationship. Alphys may become more outgoing and Undyne may become more empathetic. Also, the theme of adventure in their relationship is present in some fanfictions, where they go on adventures together in the Underground, strengthening their bond along the way.
Another theme could be adventure - related. Since they are both characters from the game Undertale which has an adventure - based storyline. They might go on adventures together in the fanfiction, facing challenges and growing closer as a result. Maybe they team up to save the Underground from a new threat or something similar.
One common theme is friendship. Papyrus and Undyne are often shown building a strong bond of friendship through various adventures or just daily interactions in the Underground. Another theme is mutual respect. They both recognize each other's strengths, like Papyrus admires Undyne's fighting skills and Undyne respects Papyrus's determination.