hmmm so its not really confirmed. i looked thru the author's twitter and the q&a they answer and i didnt find anything about a new character added to this. there was a separate tweet about a new character they are planning for their next work though - for which they shared some images. who knows if someone chatgpted all that into whatever that was, and posted it on bato. ive seen people claim (fake) things that are in the future chapters for original stories that are still ongoing, and for which there is no way to know unless you are the author yourself.

Author stated that for the side stories, there will be a new guy that comes in, a new ML. He is the guy that was taught by Seolhwa to be a good civil servant. The new ML is apparently someone powerful and good that cannot be touched so easily by the ML.
However, rather than a ML, I see it more as a third party to stir things up. Author said it is to make the ML jealous and possibly realise his feelings for Seolhwa.
One speculated that the new guy could be the true ML who saves Seolhwa. But that is highly unlikely unless the author wants to do a plot twist + it seems like the author likes ML.