When Yuuka scoffs at Nana’s barb, Nana tries a different tack, stating “Humans don’t believe the truth. They believe people.” Even if Yuuka knows what Nana tried to do, Nana has the trust of the others, while Yuuka is someone who screws her dead boyfriend. So Nana proposes a deal.
She’ll keep Yuuka’s dirty secret and even read her beloved Shinji’s thoughts if Yuuka releases her. Because Yuuka is “a hopeless romantic” she falls for the ruse, and Nana says Shinji is thinking “Kill Nana”, which Shinji later confirms is the true, though Nana was again simply bluffing. Bottom line, she gets the hulking Shinji off her back and flees into the woods.
During the ensuing nighttime game of cat-and-mouse, Nana gains crucial intel on Yuuka’s powers. Yuuka needs a”relic” (an item the deceased has touched) to raise someone. This culminates in her having a whole pile of weird stuff under her bed, and Nana is chased not just by Yuuka and Shinji, but a whole zombie army composed of dead students and teachers.
The presense of so many dead people ont he island suggests that the “Committee” Nana serves didn’t tell her everything, but she’s too busy trying to survive to give that the thought it deserves. That brings us to Yuuka’s other weakness: her zombies, including Shinji, only “work” during the night.
Nana proves that theory by putting her life on the line, locking herself in Shinji’s cabin where zombies are lying in wait. They fall asleep once the sun rises over the horizon, so she’s safe for the time being, but then Yuuka locks her in the cabin for the day, content to finish the job once the sun is back down.
When Yuuka and Shinji return to the cabin after going to class (during which time Kyouya mentions how the clasp of Yuuka’s necklace is damaged and loose), Nana is nowhere to be found. There’s a nice double fake-out when I figured Nana was pretending to be the zombie girl with a similar hairstyle, but that was a red herring.
Nana not only managed to escape from the cabin, but gets the jump on Yuuka as she’s carrying a dormant Shinji during the next sunrise. Then she pulls the real relic Yuuka uses for Shinji from her back pocket. The necklace was a decoy; the scrap of a test is the real thing. And now that it’s in Nana’s possession, Yuuka is in her pocket.
Nana explains that while it looked like she was panicking and barricading the door to the cabin to seal herself in, in reality she was removing screws from the lock, then covering it with tape so Yuuka wouldn’t notice. That’s why she was able to escape the locked room…it wasn’t really locked. She then replaced the screws before Yuuka and Shinji returned.
When Yuuka offers to do anything for Nana, Nana is perhaps a bit too cruel in saying “I’m a cat person…I don’t need another dog,” referencing Michiru. She then has Yuuka return her needles, but drops the scrap of paper off the cliff anyway. With no more relic, Yuuka will be unable to revive her undead boyfriend…he’s gone forever. But that’s not the end of the story.
Nana is curious why Yuuka chose something so flimsy for a relic for someone so important to her. Breaking down the details of Yuuka’s story about their movie date and ensuing fire, she determines that Yuuka and Shinji were never boyfriend and girlfriend; that Yuuka was merely a stalker, and she set the fire to kill the girl he was going with.
This sends an already crazed, despair-ridden Yuuka fully off the deep end, but Nana doesn’t bother pushing her off the cliff. Instead, she kills her with a needle, and the phone confirms she saved 500,000 lives by doing so. Her greatest threat thus far has been eliminated, but in the process a lot has been revealed about just how little she knows about the people who hired her. She could well be struggling and fighting for the wrong side!