Fairly upfront? Sunhwa is literally putting on a fake personality to Nanjo, leading Nanjo on, sleeping around, and telling everyone but Nanjo he's just in it for the money
Literally
Oh maybe i'm not remembering correctly bc i did skim quite a bit in the beginning. I recall Sunhwa acting a lot cuter/sweeter to Nanjo, but I dont recall him trying to lead Nanjo into a fake relationship or expressing any false feelings/promises of exclusivity.
My understanding is that Sunhwa and Nanjo were sexual partners, and while Nanjo was soft on him, Sunhwa basically just kissed up to Nanjo so he'd get more monetary gifts. His personality is a bit fake but nothing of real consequence if Nanjo is giving out gifts without expectation of anything further.
I came into this expecting Sunhwa to be a major problem, just a tool to get the relationship going, but honestly he's not bad. He has his own social circle, is fairly upfront, and even cares about others like Mokyeon. It's very much just about the physicality for him and i like that he's honest about it.