I know your comment has some dislikes, and people may see it another way, but I genuinely think that you have a point to make, specifically about how Rian can be so misleading and deceitful towards Cadgan and Hector both. 

(spoiler warning for the story in both comments btw)

~-Understanding Rian-~

- By me -(o.o)-

I will expand more on it. Please hear me out: In the sense that I, too, think that Rian is at fault for many misinteractions as well. At first, I thought his dialogue was sweet and endearing. I was starting to think being 'TeamRian/Ranzkai' was the way to go about it... But, the more the story advanced, the faster I realized he IS lowkey a bit messed up. Even if he didn't mean for it to come out that way, the things he says and does are suspicious, and I truly get what you think, but then, and here comes my thought about it...

I mean, who wouldn't be at least a little messed up on his shoes to begin with? I think that we first need to understand how it was for him from the moment his existence began; all he knew was darkness, and then, when he gained a semi-transparent physical form, he held nothing other than the past disgust, wrath, rage, and fear Cadgan made him hold. I mean, what other choice did he have? None of course. Then came the years together with Cadgan alone. Rian learned from the memories of the past Cadgan, and surely he could soon speak and do stuff for himself as he possessed control of their body, but things were quick to become different as someone else got into the picture, and that being, of course, Young Hector. 

For Rian, at least at first, he was just another small, unknown boy. But as Hector kept trying to get to know him and get closer, Rian found himself observing the badger with caution and something else he couldn't name. Because he never felt it. Nor did he knew he could ever "feel" more than the stuff Cadgan led him carry. Days passed, and in between walks around the manor of the duke, and medical examinations/visits, they would converse more. At this point, Cadgan, of course, would soon start pretending to be Rian once it was his turn with the body. This "Clinical Letargy"  illness was more so by the fact that their mind, both Cadgan's and Rian's, were severed, unwilling to commune together in the same body. Cadgan refused him and hated him after all, back then, of course. A few days passed, and finally, the walks and hangouts on the lake happened between Rian and Hector. 

Right up to that point, they discuss more about their past. Rian would get flashes of Cadgan's memories sometimes due to their shared connection, and then he would become too sad and start crying each time; he didn't know how to control it, of course. It was then that Hector would proceed to comfort him with his calm and quiet presence, and just then, we read about how Hector meant a lot to Rian in those little moments. And the moments that would follow afterwards. And then those moments, too. They start playing together, Rian teases him sometimes, and Hector would humor him gently. 

And then we get lines like, 

When he wishes Hector's words weren't so shy,

Hector says to Rian that he can have a place beside him if there is none for him in the world. And the deciding one from Rian to Hector in his thoughts, something like;

"When Hector is around, the world makes sense, and the question of whether I'm Cadgan or Rian stops mattering. Because whatever I am, it might be alright if he's near me. If we can keep laughing, and make new smiles, then maybe I won't need to yearn for old ones anymore..."

They would then pass on to become close. Hector would start feeling things for Cadgan, thinking he's been hanging out with him all along. This wasn't true for the most part, Cadgan didn't really think anything too romantic when hanging out with Hector.

And then months passed, and as Hector left to accompany his father, Rian never forgot Hector. In fact, he seemed to acquire a fascination with the Badger. It would then turn into ramblings about him, the sent letters, and everything. 

I can take all of this as evidence to prove that, not only Hector, without meaning to, helped Rian understand that there were times he could feel at peace, but also proves that Hector would go on to become his sole, truly good experience ever since he began existing. The moments with Hector marked him for life. 

If you take all of this into account, Rian wanting Hector all to himself years later starts making way more sense. Curiously, one of the only times we see Rian speaking with the use of "Us" or "Our", that is, including Cadgan too, was when Rian said to Cadgan, "Our boyfriend," referring to Hector, so that's that...

Conclusion;

 I think deep down, Rian fears that Hector would start loving Cadgan too much and drive all the attention away. And on the contrary, that Cadgan would also love Hector back even more.

I needed to show my understanding so far, at least from my point of view. 

Thanks for reading, love y'all that beared with me in this long ass message😭😭💕