Oh, so many dislikes. I understand why many people hit that button, many see that relationship as """"cute"""", but I see your point. Maybe you are right, but we need more context. I'll explain by points:
SPOILERS AHEAD
1-- Rian was born from what Cadgan rejected: fear, sadness, love for being a priest, and rage (but later Cadgan recovered that rage and redirected it toward Rian while he was still kidnapped by the duke). So those were the only things Rian could feel. When Hector appeared, they started seeing the world in a different color, and all of his fears disappeared, so his love was born from all the negative emotions he carried.
"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."
2-- Continuing the previous point: Rian doesn't want to lose Hector. Hector is like a patch, or better said, a drug that covers his pain. That's why he is so obsessed with him and acts very possessively, using "I/me" when talking about Hector and rarely "we/us." He does use "our" once when calling him "our boyfriend" while they were spying on Silvareth, but the context is a little tricky: that night it was actually Cadgan who was with Hector, not Rian. So Rian is using Cadgan's intimacy with Hector to include himself, and claims him as "ours" only when it's convenient.
3-- Cadgan is more cold-headed because he doesn't carry all those emotions, making him more reasonable than Rian, but also less emotional about Hector. Cadgan doesn't see himself as worthy of love, or doesn't see it as important enough, which is why he tried to reject Hector. He is probably denying his own feelings, or at least letting Rian love Hector for both of them. So we can say Rian loves Hector freely precisely because Cadgan is rejecting his own feelings, but he never consciously passed that feeling to Rian like he did with the others before. Also, Rian was trying to convince Cadgan to be with Hector (because he knows that Hector remembers Cadgan's face), but Cadgan didn't show much interest, either out of denial, or because he genuinely doesn't know how to act in those situations.
4-- We can't forget that Rian changed physically, but Cadgan didn't. Hector remembers Cadgan's face but has never seen Rian's, so that's likely why Rian uses "I/me" all the time. It may be his way of affirming his individuality apart from Cadgan, and his desire for Hector to recognize him purely by personality.
5-- Last but not least: we don't know how the author views relationships, whether he sees them as traditional or open. The novel establishes that Cadgan and Rian are the same person, but also that they are different. So Hector loving both of them is, in theory, a polyamory relationship, just like Venom and Eddie with Anne. Maybe when the novel is finished, it'll end with Hector with just one of them... or both.
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I'm not saying Rian is manipulative, but he is childish, throwing tantrums whenever Hector is involved. He's like the kid in the neighborhood who says "the ball is MINE and I decide when YOU play", except the difference is that Rian doesn't even realize he's holding the ball. The love Rian shows doesn't seem healthy anyway.