Okay, so the Healer Han scene in Kpop Demon Hunters is referencing/drawing a lot on concepts of systemic psychotherapy.
Sounds like a big word, but it's really a pretty simple concept. Basically, systemic psychotherapy is a therapy that aims to help people work through their maladaptive, toxic, or less-than-useful habits by addressing the context of the person's close relationships and the system they're living in. It assumes that a person's problems aren't happening in a vacuum.
So like--think of a standard family unit. Parents might bring in a kid and say, "this is our problem child, they're always acting out, they need therapy." In the systemic psychotherapy model, the therapist won't approach this situation as "there's one bad apple in this family and everyone else has no issues." Instead, they'll assume that the identified "problem child" is reflecting a deeper dysfunction present in the overall dynamic of the family unit. They will approach therapy with the child with the mindset that for the child to be able to change their behavior, they will need to alter the relationship dynamics they have with their other family members, and therefore, the other family members will likely need to alter their habits and mindsets as well.
There's this general concept in systemic family therapy, which I think could be extended to the huntrix girls, that each member of the family plays a certain "role" in the family's dynamic, adjusting to attempt to keep their group dynamic stable. For instance, the family member most often labeled the scapegoat or the problem child is usually the family member who's trying to call out the underlying issues in the family. They're the person who wants to fix things directly. But they're not the true problem--they're just the person who's calling out the problem. Meanwhile, other family members are likely addressing The Problem from other angles. There might be a "healer," someone who quietly picks up the pieces of whatever gets broken and tries to smooth things over with everyone after big fights. There might be a "golden child," someone who is uplifted as The Example and often ends up taking on much of the burden of keeping certain members of the family happy.
...I mean, there's a lot of different possibilities. But the point is, all these dynamics work together. They don't just exist in a vacuum. They're like cogs in a machine--moving one moves them all, and vice versa.
So when Healer Han tells the girls:
...This idea of systemic dysfunction is pretty clearly what the movie is tapping into.
Han identifies Rumi as a workaholic who has a lot of walls up, Zoey as a people pleaser, and... well, Mira sort of hijacks her "therapy session," but that's kind of the movie's way of making it clear what her issue is. She uses aggression to cover up her vulnerabilities.
This scene is not just a good setup for what each of the girls need to address in their emotional arcs, it's setting up the idea that each of the girls' emotional blind spots and issues are working together to create a fragile and susceptible group dynamic. Despite the fact that they love each other and are each trying their best, the girls' friendship has problems at the beginning of the movie. Rumi is terrified of confiding in Mira and Zoey. Zoey is constantly trying to prove herself and be easily likable for Rumi and Mira. And Mira gets extremely insecure to the point of lashing out with harsh interrogations whenever she feels that one of the others is pulling away, being distant, or keeping secrets.
That's not, like... the healthiest. Obviously the girls' relationship has many good parts, but it's also got these huge blind spots that become a really big deal by the end of the movie.
The point this scene is making is that Rumi's problems are not happening in a vacuum. And neither are Mira's, and neither are Zoey's. Their problems interact with each other to create their group dynamic. Rumi's avoidance and secret-keeping makes it hard for her to genuinely reassure or work with Mira on Mira's insecurity around non-transparency. Mira's insecurity around non-transparency makes it hard for her to confront Rumi about keeping secrets in a healthy, gentle, loving way--and beyond that, her aggression clearly starts getting to Zoey, who as a people pleaser is super uncomfortable seeing her loved ones fight. And Zoey's people-pleasing tendencies and her commitment to not rocking the boat leads to her avoiding the issues building up in their friendship until they've built up to the point of eruption.
In order to move forward and be healthier, all three of them need to change. If just one of them changed their behavior, it wouldn't actually fix their overall relationship, because all three of them are reacting to each other and adjusting for each other. Each of them has genuine, practical reasons for acting the way they do within their shared dynamic--but just because they have practical reasons for it doesn't mean that the way they're acting is emotionally healthy or productive.
This is why What It Sounds Like is not just a turning point for Rumi, but for Mira and Zoey as well. Each of the girls needs to learn to sit with vulnerability, with uncertainty, and with discomfort. All three of them. This is not Rumi coming back to Mira and Zoey with a perspective shift--this is all three of them coming together and having a shared revelation that Rumi initiates.
To fix the part, you have to fix the whole.