If someone asked me to sum up their dynamic in a phrase, I'd probably say it's a long, messy lesson in learning to see the person right in front of you, instead of the ghosts they remind you of. They start off so entangled, but not really connected. It's brutal and real in a way I haven't seen in many other shows.
Early on, it's just mutual convenience tinged with resentment. Hanabi is using Mugi to feel close to her childhood teacher, and Mugi is using Hanabi to stay connected to his music teacher. They're mirrors for each other's impossible crushes, and that creates this weird, cold intimacy. They know each other's secrets, they share this physical space, but they're looking through each other.
That middle phase is where it gets complicated, in my opinion. The jealousy starts to creep in, but it's confusing. Is Hanabi jealous of the women Mugi sleeps with, or is she jealous that he can move on from his obsession while she's still stuck? There's a scene where she tries to copy one of his casual partners, and it's just heartbreaking because it shows she's starting to care about his gaze, not just her teacher's.
By the end, after all the external relationships implode, they're left with just each other. The final scene on the train platform isn't a declaration of passionate love; it's an agreement to maybe, possibly, try building something real from the ashes of what they used as a coping mechanism. The evolution is from utility, to confusion, to a fragile, hesitant potential. It feels less like romance blooming and more like two people finally sobering up after a long, self-destructive binge.