One way is by having them overcome their differences. Yeon may have a more brash personality while Bam is more reserved. By working through these differences, like when they have to make a joint decision in a life - or - death situation, their relationship can develop. Also, they could be put in a situation where they have to rely on each other's skills. If Yeon needs Bam's climbing ability and Bam needs Yeon's combat skills, this mutual dependence can enhance their relationship.
Common themes might include their friendship evolving in unique ways. For example, they could face challenges together that test their bond. Another theme could be their growth as characters in relation to each other. Yeon might influence Bam's view of the Tower, and Bam could inspire Yeon to be a better version of himself.
I'm not sure of specific Bam Bam Brown fanfic, but you can try looking on fanfiction platforms like Archive of Our Own. They often have a wide variety of fan - created stories for different topics and characters.