I think having RO’s with an involvement in the plot, however that may take form, is a good opportunity and allows for divisive conflict or heart-warming moments of camaraderie. But my personal opinion, is that I would prefer a RO who is relevant to the story as a whole, but who’s involvement in the plot is developed on more through their involvement with the PC.
So this may take the form of shaping how the PC interacts with the plot as the PC is concerned about how it may effect their relationship, or the RO. I feel that instead of having the RO be linked directly to the plot, have them be indirectly linked through the actions of the PC.
For an example, you may be a superhero sworn to defend the city of Faketropolis, and when under the guise of your secret identity, you are in a healthy relationship with a police officer. During one fated fight against a villain, it becomes clear that the police and your RO is going to be put at risk by the attacks and the scenes that play out will have a far reaching effect on your relationship.
Do you be merciful and focus on defending civilians which is what your RO would prefer. Or do you focus on taking out the villain to stop future attacks, which may risk many civilians lives and possibly even your RO’s life.
I think by having the existence of a RO influence the moral choices that a PC makes, and having some choices directly surround or be impacted by the RO is a far more believable relationship and one, that in my opinion, makes for better reading as I feel more invested in the PC’s relationship as it feels like a living and breathing part of the story.
This may additionally, if the author were writing the book, allow for the RO to eventually become integral to the plot through the PC and to perhaps even diverge from the PC to become an antagonist without it being forced from the very beginning.
Of course, these are just my thoughts and regardless I thoroughly enjoy reading anything even if it doesn’t fit this ideal.