I love how this author leaves some ambiguity to the line "That's definitely not love." It could refer to Heebum's feelings towards YooWon or vice versa. Therefore, in order to get a better understanding of their relationship, Jumi comes in and acts like a driving force for both of them to reexamine their feelings. By the end of "that summer," they would likely come to a realization about themselves as well as the thing called "love."
P.S: I'm not saying that Jumi is a driving force that pulls them towards each other again. Heebum could still realize that his feeling towards YooWon is actually dependence/yearning/obsession (anything but love) and that he loves Jumi romantically or no one at all.
This ship is Titanic but I still hop on it with no regrets (/TДT)/
same ^^