Responses
I think even more than that isagi and nagi relationship is like Sae and Rin were Isagi is the prodigy(Sae) that inspire the genius (Nagi) but instead of The prodigy being broken it's the genius and we know from the itoshi brothers or even ego and noa how important that relationship is
As for the isagi demon king title I don't believe it really refers to all the arc of isagi but really nel isagi since it was God chosen emperor (Micheal Kaiser) vs Demon king (Isagi)
The difference between Nagi and Isagi's development despite them working for a similar goal shows so much about them. Nagi doesn't push Chigiri to go beyond his limits and end up possibly injuring himself permanently but Isagi does and even encourages that mindset. Isagi is being someone that would use others with little to no remorse for their dream and is being more and more of a "demon king".
Anyway. Back to Reo and Nagi. In the recent BlLk chapters, Reo scored a goal and immediately apologised to Nagi but Nagi told him he didn't need to apologise and it was his fault for not being fired up. Nagi dell of the leader board and might he expelled from blue lock.. moreover, this chapter shows Nagi seeing Reo in Luna, but other fans who read Blue Lock made a theory that Reo might be the second Chris Prince.. I can't wait for future chapters.