How do we know that Sungji is not just one of Sumin's "conquests"? What makes Sungji special? considering Sumin has had her fair share of the market, and she's still coming back to Seju-momma!
I'd feel bad if Sungji was just another insignificant girl in Sumin's life.
I don't quite remember every moment, but there have been hints here and there showing that Sumin has been affected by Sungji's unique personality.
1) When Sumin got drunk af on the day of her parents' anniversary, she hugged Sungji and attempted to make Sungji fall in love with her (i.e. close physical contact + "I'm so glad you're here blah blah blah"). But Sungji didn't just blush and give in to Sumin's advances. She told her what she honestly thought ("You're strong because of the happy memories you had with your parents, and I hope you can tell me about those memories next time" or something like that), and she made Sumin fall for her instead. If Sungji had given in to temptation at this moment, she probably would have been another "conquest" girl.
2) Sumin didn't allow Seju touch the band-aids that Sungji put on her fingers. So clearly Sungji is important to Sumin in some way; she didn't want the parts of her body that were...purified, I guess, by Sungji, to become tainted by Seju.
3) She starts doing stuff that she normally would never do, like buying dolls and putting out her cigarette when talking to Sungji on the phone. Although old Sumin was a romantic, I feel like she stopped being one after her relationship with the chairman. The lawyer lady broke up with Sumin because she felt like Sumin wasn't putting any effort into their relationship.
4) Sumin's attitude towards Sungji has been changing. At first, she was very skeptical, and she doubted that Sungji was as innocent as she was. However, now we can see that her inner thoughts have become less harsh. She went from "I wonder if she'll fall for me if I say blah blah blah"/"First love? Yeah right"/"Her heart's racing lmao", to "I wonder if I'm supposed to just say that I'm thankful for her effort"/"We can take it slow". She's actually trying in a relationship. And Sungji has made Sumin less of a grumpy asshole.
5) From what we've seen, most of Sumin's "conquests" started out as sexual relationships. Sumin and Sungji still haven't slept with each other. Sumin tried to sleep with Sungji in the anniversary chapter and she failed. She got drunk with Sungji again for her birthday, but chose to not sleep with her this time. Sumin is putting time and effort into this relationship, even though there hasn't been any sex involved so far. So I'd assume that something's different.