Don't think the author is trying to portrait mister as a second ml (at least, not for now). Although his mere presence does not pleases seo-in at all. Unlike his father who is violent with yeongwoon, he's a good masculine figure for him. I think this brings up seo-in insecurities because, as he himself said it, he wants to be the only person for yeongwoon (kinda reminds me of how cirrus in lost in the clouds, wanted to isolate skylar to get closer to him).
Don't think the author is trying to portrait mister as a second ml (at least, not for now). Although his mere presence does not pleases seo-in at all. Unlike his father who is violent with yeongwoon, he's a good masculine figure for him. I think this brings up seo-in insecurities because, as he himself said it, he wants to be the only person for yeongwoon (kinda reminds me of how cirrus in lost in the clouds, wanted to isolate skylar to get closer to him).