In krystal x human male fanfics, the theme of 'power and vulnerability' is often explored. Krystal may have certain powers or abilities, and the human male may be more vulnerable in comparison. But as their relationship develops, they both learn from each other's strengths and weaknesses. Additionally, the theme of 'sacrifice' can be seen, where either character is willing to sacrifice something for the other, which adds depth and drama to the story.
A common theme is self - discovery. The male character might be going through a journey of self - discovery, and Krystal plays an important role in that. Also, the theme of 'finding a home' can be present. The male might be lost or alone, and with Krystal, he finds a sense of belonging. And of course, the simple theme of 'falling in love' is always popular in these types of fanfics.
One popular theme is the 'forbidden love' concept. Since Krystal might be from a different world or have a special status, a relationship with a human male could be seen as forbidden. Another theme is 'overcoming differences'. They may have different species - related or cultural differences that they need to overcome to be together.