In the first chapter, Hisashi died and turned into 'them'. However, what i don't understand is why did Rei say that she wanted to become 'them' too just because Hisashi died. I mean she still has Takashi, the one she claims to truly love. When Rei said that she wanted to turn into 'them' too, it basically meant that she loved Hisashi more than Takashi as she wanted to follow him to the death. I mean if she loved Takashi more than Hisashi, she would never say that. She would want to live on and be with Takashi. In addition, she went as far as to tell Takashi that he must have felt good that he killed Hisashi (even though Hisashi was about to eat her) because he must have hated him as they were dating. This right here is the proof why Rei can never claim that she has strong feelings for Takashi, if she did then it would be a load of bullshit.
Also, after I have read a few chapters, after Hisashi's death, Rei seemed to be trying to get together with Takashi. I mean she ignored him when Hisashi was still alive and now that he is gone she's trying to talk to Takashi like nothing happened. Like she never abandoned her promise to Takashi to get married, which made Takashi go through hell. This led me to think that Takashi was Rei's second choice as, if Takashi was not Rei's second choice then the two of them would have dated each other. Furthermore, when Rei said that she told Hisashi first about the reason regarding her staying in the same year, led me to think that Rei does not trust Takashi enough. Otherwise, she would have told him first.
To conclude this, Rei has never shown strong feelings for Takashi throughout the manga, and the anime. She's only trying to get him now because Hisashi died. Other than that if the zombie apocalypse had not occured, i assume that Rei would have ignored Takashi still and continued to date Hisashi