Second Mission [1]
What are the qualities of the moon and why was it able to be classified as its own affinity?
In truth, the latter half of the question could remain unanswered for eons if nobody looked into it. The reason was simple, the sheer number of affinities and elements existing was too large,
Although most people are born with the average affinity towards one of multiple of the most basic elements, it wasn't that rare to see someone with a unique affinity.
Something like Ruyue's moon affinity, however, was especially rare. It was because of how specific it was more than anything else.
In fact, even with her yin affinity, Ruyue had the ability to manipulate lunar energy. It was a facet of yin's connection to all things cold or negative.
However, she was specifically born with an affinity toward the moon as well. It was something that should grant her a less streamlined set of abilities, but even now she hadn't been able to understand it.