Yao Long was one of the former monks in charge with protecting the Nexus and one of the original Yo Kwon Do fighters. However, he became corrupted by darkness and wished to unleash and manipulate chaos. He was then sealed within his yo-yo by his former colleague, Lao Shi, after the latter destroyed the Nexus.
It is later revealed in The End, so far... that he manipulated Belloc to collect the various dark shards to reconstruct the Nexus' dark half in order to free himself from Belloc's yo-yo (which was Yao Long's) making him the hidden main antagonist of the series.
Prior to the series, Yao Long wished to control the power of darkness and fought Lao Shi at the site of the Nexus. Disagreeing with his partner Lao Shi's pleas for balance, he engaged combat with him, but ran to Nexus. While attempting to fuel its dark half, Lao Shi destroyed the Nexus in the process, sealing Yao Long inside it.