One common theme is friendship. Despite their different personalities and roles in the Kingdom Hearts universe, fanfictions often explore how they could become friends or allies. For example, they might overcome their initial differences and work towards a common goal.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Kingdom Hearts. Just use the search function and type in 'Kh Marluxia X Axel' to find relevant stories.
One common theme is their relationship within the Organization XIII. Writers often explore how their different personalities interact, like Marluxia's more devious nature compared to Vexen's scientific and somewhat cold demeanor. Another theme could be their loyalty (or lack thereof) to the Organization. Some fanfics might also focus on a possible romantic or platonic bond that forms between them as they work together on various missions in the Kingdom Hearts universe.
Love, of course, is a very prevalent theme. Whether it's a slow - burn romance or a sudden realization of feelings, it's often at the center of the stories. And sometimes there's the theme of redemption, where one of them has made mistakes in the past and the other helps them become a better person.
Axel shows his love through his actions. He'll protect the reader from danger, like when facing off against Organization XIII members who might see the reader as a threat.
One romantic moment could be when Axel saves the reader from a Heartless attack. He swoops in, his chakrams flashing, and then holds the reader close, looking into their eyes with his intense red ones. It shows his protective side which can be very romantic.
There are often fanfiction communities on social media platforms too. Check out groups dedicated to the relevant fandom. In these Brick x Axel fanfictions, you could see different interpretations of their relationship, like in a modern - day AU (alternate universe) where they are students in a school and their relationship unfolds in that setting.
The popularity might stem from the complex relationship between Axel and Roxas in the original source material. Fans are intrigued by their dynamic and want to explore it further in a more romantic or spicy way, which lemon fanfictions can provide.
One common theme is friendship turned into something more. Since they start as friends in the Kingdom Hearts series, many fanfictions explore how their relationship deepens into a romantic one. Another theme is the struggle with their identities. Both characters have complex backstories related to their identities, and this can play a big part in the fanfictions as they navigate their relationship.