It's possible that the 'rate' at which a girl falls for Rokudou could be tied to how bad she actually is. Minami is a mean, violent miser, who has no problem with ruining other peoples' lives for the sake of selfish desire; operating within the law also means nothing to her. She's also already obsessive, and this trait makes her fall even faster. It's arguable that Tsubaki was the nicest bad girl, since it took her quite awhile to finally realize her feelings for Rokudou; however, since she effectively became 'good' in the process of getting to know him, his curse no longer seems to be affecting her. The same could hold true for the shrimp, though her feelings were more muddled.