It turns out Yoo-mi has one impressive quality that Jin-wook is unlikely to find anywhere else. She's good at apologizing. The irony, of course, is that Yoo-mi hasn't done anything wrong in regards to Jin-wook or his relationships. All the same, at the slightest indication that she has made either Jin-wook or Hye-ri uncomfortable, Yoo-mi demurs. And of course, Jin-wook is left in an even worse mood than usual as a result.
I like the difficult position Jin-wook has been put in. His personality is naturally abrasive and he does stuff all the time that would actually justify a person demanding an apology out of him. So Yoo-mi's ability to be so incredibly passive that it makes Jin-wook feel ridiculous is a good thing. She can make Jin-wook slow down for a minute and appreciate life, or at least the fact that other people are in it and trying to make a positive go of the experience just like he is.