I-330 alters D-503's entire perception of his life. He becomes obsessed with her and starts to become more aware of his inner life, something he repressed for the benefit of his subservience to the One State before. As a result, D-503 is no longer his old self, and other characters begin to take notice, particularly his sexual partner O-90.
In the One State, individuals are assigned sexual partners to relieve their urges with, and these partners are usually shared with other people so as to not foster jealousy and to prevent people from forming bonds that might become more important than serving the state. O-90, though assigned to D-503 as his lover and, from all appearances, a dedicated conformist to the rules, is in love with him. Though love is forbidden in the One State, O-90 initially has no issues, since D-503 does not harbor passions for anyone; however, once he is in love with I-330, O-90 grows jealous.
This suggests how D-503's rebellious emotions—his jealousy regarding I-330, his feelings of discontent with an overly rationalized perception of existence—are being transferred to other people in his life, almost like a sickness (considering that D-503 refers to his burgeoning rebelliousness as a sickness, this analogy is likely intentional). O-90 herself eventually rebels against the One State by bearing D-503's child and raising it on her own (rather than giving the baby up to the One State). Had D-503 never met I-330, O-90 might have continued living in accordance with the One State's wishes.