The more I thought about this, the more I realized just how much thought is probably behind your deceptively brief "obsessive desires and mutual codependence"...
I think it's fair to say that Mahiru and Kiku's relationship was characterized by codependence: unlike Kou, Mahiru already knew how to love and wanted to become the underling of the Kiku he loved, but that love came from the idea that she was all he had (ch. 104, p.14). Furthermore, Kiku wanted that he cut off his friends and family so that she could come to love him... so that she could (hopefully) die from vampirizing him.
Kou and Nazuna's relationship is generated from an unlikely reciprocity that defines, permeates, and actively develops their relationship. Kou wanted to learn to love (in order to become a vampire) and Nazuna wanted to drink his blood until he turned. In contrast, what Kiku wants practically obviates what Mahiru truly wants-- to be with Kiku (ch. 104, p.15) and he only has it in him to do what she wants out of his love for her. They lack anything like the reciprocity that Kou and Nazuna have-- they lack reciprocity, period, and it's because of this that I find it difficult to say that they loved each other. Certainly, they had a dysfunctional and mutually destructive love.
Furthermore, Kou and Nazuna's relationship led to both of them making new friends and growing from those new connections, also meaning that Kou got the chance to reconnect with his friends, reconnect with his mother, make amends with the girl he rejected (and her friends), and overall overcome the initial social withdrawal that characterized him at the beginning of the story. Not only does their relationship help Nazuna make amends with Kyouko, Kou nearly singlehandedly saved Kyouko's life and her future. Both Kou and Nazuna start out this story knowing nothing about love outside some fragmentary preconceptions, and the fleshing out of their understandings of love (as well as their own coming to love) is on account of their various observations of and experiences with romantic and platonic relationships.
Kiku and Mahiru have no such experiences, and thus have no such developments.
Kiku's wants necessitated Mahiru systematically withdrawing from society, cutting off his mother, and nearly definitively cutting off his friends by the end of his life (definitively, if you read into him crushing his phone that stored the last preserved memory he had of him and his friends)-- and if not for the fact that he loved Kiku, he wasn't fond of any of that, but still elected to do it because Kiku was all he had... but that's partly because of Kiku herself.
Kiku didn't understand love, and Mahiru didn't understand love despite easily coming to it. They cut themselves off from anybody who could show them any better-- or at least show them something different. Furthermore, in cutting themselves off from people, they were freer to engage in more self-destructive activities and entertain more self-destructive thoughts.
The end result is that, by the end of their relationship, Mahiru and Kiku come to some kind of mutual love, but it's the product of the same preconceived notions they had at the beginning of their relationship-- at most, Kiku had her tragic romance films, which either informed or reinforced her internal dichotomy that had her wishing for a happy life despite pursuing something she figured would lead to her death, and may have also made her understand someone sacrificing all their relationships for her as something "romantic". Mahiru-- an impressionable teenager that probably didn't know anything about love outside his feeling of it-- only got to inherit that.
Another comparison: Kyouko failed to become a vampire, but the life she led after her parents' deaths wasn't far away from the tedium of a vampire's life that Nazuna described in chapter 42, as well as the suicidal tendencies inherent to Kiku's machinations. She lived by herself, in an absolutely barren box of an apartment (ch. 83, p.16). She had no friends and no associates, and had a grand plan to reveal the existence of vampires that involved her being murdered by one. Even when that fell apart, she had to be prevented from killing herself. In general, she lived to hunt vampires and die trying.*
Some of the ways she's treated Kou (a 14-year old) have been... "downright suspect", for lack of a better word (not that you'll catch me complaining), but she deeply appreciates his friendship because it was what saved her. And it's because of that friendship that she found more friendships, and was taken even further away from that nadir.
If Kiku found what Nazuna had-- multiple people who, more than being charmed by her, were willing to have enduring and mutual relationships with her-- and she sought to keep them, she might have been more willing to live her life rather than constantly seek death.
*Both Kiku and Kyouko had the means to kill themselves at any time (Kyouko could have shot herself and Kiku could have stepped out into sunlight), but they seemingly both wanted to die in some kind of "human" pursuit.
