Ah... a mysterious, very powerful individual with great influence on the world of this novel has appeared. Fifty-fifty chance he’ll become a ‘love interest’ (it’s common for these types of stories to eventually throw in a over-arching love interest, otherwise it will never end, but the odds aren’t exactly in Ghost King’s favor) and eventually help MC and ghost hunter girl defeat the old evil master. Either that or he is a neutral party that isn’t particularly bothered by who wins, as long as it doesn’t compromise his own agenda. The author is just making him too mysterious not to be worth anything to the plot. He is also likely how MC gets a perfect score by successfully ’passing on’.
Unless the plan is to go full shonen action for this arc, which is a tad far fetched, he isn’t likely going to be the main antagonist.