I hope not. Really not a harem kind of guy. Not to mention the fact that MC and Si Eun are cousins...not too weird since they're probably far enough apart genetically, if you want to get into the technicals. Nonetheless, I'd rather see them be just the familial duo. Something like, she dotes on him like a sibling in a way that's fitting for her cool personality, but sort of acts as a wingwoman for MC and Ha Neul once she "gets the memo".
Might just be me, but I'm tired of the "dense protag gets all the girls because cool and dense" trope. I wanna see "dense protag is dense to every girl except for those of the girl he's actually interested in, and 'every girl' is only extras and not supporting roles" be more of a thing. MC being good at everything except being loyal to one romantic interest always repels me from these types of stories.