When Natasha enters the store, she's surprised to find Charlie behind the counter. When she greets him, Charlie slams his phone down and takes a second to recognize her. His face and knuckles are red and swollen, and he sneers at her before making a joke about her coming back for the "better brother." She asks him for Daniel's phone number, and agrees when he asks if they got in a fight.
Approaching Charlie (or Dae Hyun, for that matter) sets Natasha up for embarrassment or heartache or both, so the fact that she's willing to do so shows just how much she values her connection with Daniel.
Charlie smirks, plays with his phone, and asks seriously if Natasha has a fetish for Korean boys. She insists she just likes Daniel, even when Charlie scoffs that boys like him are easy to come by. Natasha realizes that Charlie hates Daniel because Daniel clearly doesn't hate himself—he's more comfortable in his identity than Charlie ever will be. When Charlie asks why he should help, she reminds him that she'll absolutely get Daniel in trouble with their parents and he'll be able to watch and laugh. He does laugh, and Natasha thinks that Charlie has very few good parts to him. Regardless, he gives Natasha Daniel's number.
Natasha's realization about why Charlie is such a miserable person draws upon the intersection between isolation and the immigrant experience, as Charlie's misery stems from his attempts to cut himself off from a major part of his identity. Daniel, on the other hand, is able to exist as a whole and happy person because he accepts both the Korean and American parts of him. Again, this shows that there's no right way to be an immigrant, but it's easier for some than for others.