She began her career as a doujinshi artist creating doujinshi for works such as Saint Seiya, Captain Tsubasa and Maouden.
She debuted in a commercial magazine with the original work Metal Heart (serialised in Kobunsha's Comic VAL from November 1986). After her debut, she continued to contribute on numerous doujin works. However, the later disbanding of a doujin circle, Yajou Teikoku, she had founded with Maki Chikura, led to her taking numerous breaks from her commercial works. Due to this, there were incomplete works penned by her during this period.
During her years in middle school, she had been a fan of Masami Kurumada's manga series Ring ni Kakero, Fuuma no Kojirou, Saint Seiya and B't X (serialised in Weekly Shounen Jump), sending Kurumada numerous fan letters asking to meet him, after which Kurumada finally agreed, inviting her to his workplace.
When asked about her penname in an interview in the September 2006 issue of Puff, she responded that she had originally thought of making it Kouga Jun, but subsequently changed it to Kouga Yun.
Among her noted works is Loveless, which has been serialised in Monthly Comic Zero Sum since 2002 and is ongoing, as well as her work Tenshichou (天使庁). She has also worked as an illustrator on the noted literary magazine Faust.
In 2007, she contributed the character designs to the Sunrise anime series, Mobile Suit Gundam 00.