The recent thread on the RS had me thinking, and I noticed something that I think makes it substantively different from the other three throughlines: the relationship has no agency.
In other words, the MC has a Concern and they are trying to do something about it, as evidenced by their being a Do-er or a Be-er.
The OS has the Protagonist who is actively Pursuing their Story Goal, and the IC also has their Concern. I’m sure we can find examples where the IC is not actively doing anything about that Concern, but nevertheless, it seems like the could if they wanted to since the IC is an identifiable person (usually).
But the RS doesn’t have that kind of agency available to them. The recent advice has been to treat the RS like a character, but it’s a character that doesn’t have access to a mind that can motivate change. The RS can certainly be caught up in Desire, say, but it doesn’t have a way to go out and pursue something better.
Or does it?
Anyway, I think this is worth talking through and may lead to some insights.