Luca Negri's Links
Since Negri despises 'god' of all sorts and being a doll to many means being less than a human, he perceives his resurrection as an ultimate form of humiliation. Even though Vanya doesn't try to control or see Negri as his possession, his obscure intention about bringing Negri back to life takes part in worsening the two's relationship.
Being the sole person who's responsible for Negri's well-being, Vanya is protective over Negri against offensive takes about his identity. Though at some point Vanya did state that he resurrected Negri because he needs an assistant for the work, Negri still holds his doubt and distrust towards Vanya as he can sense that the man knows more than what he said.
Negri feels resentful about his origin knowing that Nicholas is a psychophage and also Vanya's mentor who seems to always advocate for his protegé - something Negri never experienced throughout his former life. Yet he needs Nicholas' answers about his identity as a half-blood psychophage. He tries not to let Nicholas see that he cares, though deep down he's aware that there is no use faking detachment to something that has been existing for eternity.
good friends in general but at the same time Luca both feels grateful and fears Electra so much he never thinks of putting up a fight with her even if given a chance.
the two shares a rocky childhood so Electra can relate to Luca's hostile attitude towards people. will protect him whenever she can but also won't hesitate to smack him in the face if he does something stupid.
witness of Luca's murder who later reported it to Vanya. the reason behind Eurus stayed irrelevant to the event was that he never liked Luca for everything he did to other people throughout his former life. also Eurus has a bit of discrimination against half-breeds that was enforced by Luca's behavior.