Until they died at around the same point two-thirds into the narrative of LOCH, Mui Chiu Fung and Au Yeung Hak served as secondary antagonists in the story. Neither one posed the threat to Gwok Jing and Wong Yung that West Poison Au Yeung Fung did, but the protagonists couldn't fight Au Yeung Fung in every battle, and the other Jin flunkies were not strong enough to pose a serious threat to Gwok Jing and Wong Yung by that point.
Hence, Kau Cheen Yan really begins to play a much bigger role in LOCH at around that time. At about 95% of a Great's martial arts ability, Kau Cheen Yan seemed to provide an antagonist whenever Jin Yong didn't want to pit the protagonists against Au Yeung Fung.