I have a theory about Strike and Charlotte. parental knows it already because we chat.
I'm going from the psychology of the two people as presented, rather than anything overt in the text, but my working theory is that Charlotte knew who Strike was when he crossed the room to chat her up, and as a woman who had grown up in a dysfunctional family herself, with a taste for drama, she was always interested.
I think they would have become very close very quickly - obviously sexually, but also massively oversharing, which creates a sudden, false sense of intimacy which can distort the perception of the relationship and contribute to the destructive codepency that characterizes what we end up seeing in TCC and onwards. They were together through some hugely significant life events, like him losing his leg etc. too, and, consciously or unconsciously, Charlotte appears to have leant on the similarities in their background to cement the bond (I'm thinking here of her reaction to his rare self pity about being 'an accident', where she cossets his nonsense, instead of challenging it like Robin does.)
Whereas, by contrast, Robin and Strike's emotional intimacy has evolved quite naturally over time, and not been artificially manipulated by either of them, nor by the heady addition of sexual intimacy. It's allowed both of them to continue to grow and develop as people, and the times they have clashed have been leaps forward in communication that have helped each of them be better people separately.
If I could shorthand it, I'd say that Charlotte/Strike is essentially two damaged people damaging each other further by sucking the life out of each other, and Robin/Strike is two damaged people who are healing and growing closer in the process, and making each other better.