Onyx is a Black Dragon and a member of Fang.
Onyx acts as the antagonist of the series.
At the beginning, he's shown captured Rose and takes her to the Tokyo tower. He's actually engaged to Rose, the necklace on her neck is the prove. After a long conversation, he convinced Rose that he Ryuji hates him, and they plan the ceremony.
In the middle of the ceremony, Ryuji stops them. Ryuji then destroys Rose's necklace, so their engagement are gone. Onyx is furious, transforming into a full dragon. Rose and Ryuji perform the "engage" so that Ryuji has the power to defeat Onyx. Using Slash Breath, Ryuji defeats Onyx.