Nana may be rid of Yuuka, but her troubles are far from over. Shinji’s desiccated corpse and all of Yuuka’s zombies remain out in the open, and Nana will have been suspiciously absent from class when two more classmates died. Kyouya is the only one whose suspicions of her she must clear, so she devises a plan, using the class gyarus as pawns.
The tan Habu just happens to be out hunting frogs and snakes to eat in order to survive (her Talent is poisonous saliva), but Habu and her friend Kaori have had a falling out due to the latter’s missing blue contacts. Step One: Nana poisons Habu, gets her phone password, then deposits Habu’s body among the zombies.
Step Two: Nana informs Kyouya, Michiru, Moguo and Seiya that Yuuka is dead and explains the circumstances: Yuuka was the true necromancer, and an EoH possessed her to chase Nana with an army of zombies. Nana used Shinji’s thoughts to convince her to stand down, and she threw herself off the cliff. She uses Moguo’s fire Talent to burn Shinji’s body and the group of zombies—among which happens to be Habu’s body.
Kyouya lets the corpse-burning happen as a practical matter, but he’s not letting Nana out of his sight the rest of the day (hence the day’s longness for Nana). When he brings up the very fair point that Nana is always missing when someone ends up dead, suddenly there’s a scream from the dorms: Kaori has been found dead.
While this would seem to clear Nana as she was by Kyouya’s side, it’s clear Kaori died while clawing at her eye. He tastes the contact solution and detects poison (which doesn’t kill him, but isn’t pleasant either), meaning her murderer could have poisoned the solution at any time. Kyouya isn’t moved by Michiru’s constant pleas for him to lay off Nana.
When he searches the room again, he discovers the odd state of the window, which can only open one way, and recalls that when he was suffering the effects of the poison, Nana opened it without any trouble, as if she’d opened it before—which of course she did. It’s a major slip-up on Nana’s part, and no doubt the result of a lack of sleep and proper time to plan her murders of late.
When the ever-loyal-to-Nana Michiru produces Kaori’s phone (unlocked with Kaori’s fingerprint), she discovers a text sent while all of them were out with Nana as she told them about Yuuka and Shinji. But seeing the phone switches on a light bulb in Sherlock’s brain: he thinks he’s finally figured it out, and warns Michiru to get away from Nana.
First of all, he realizes that Nana had Muguo burn all of the zombie corpses because Habu was among them. Nana messed up her face and put her in a boy’s uniform so she wasn’t instantly recognizable, but it was Habu. Then he posits that Nana took Habu’s phone and used it to text an apology to Kaori, so she’d use the contacts Nana poisoned.
Nana’s last line of defense is the phone’s passcode; even with her mind-reading Talent she can’t ask “specific questions”. Kyouya swats that away easily: she just used the finger of Habu’s corpse to unlock her phone. Since he’s been watching her all day, he suspects she still has the phone in her pocket, which is how she sent the pre-written text while they were away from the dorms.
After Yuuka’s status as a worthy adversary fell apart due to her emotional attachments and general mental instability, Kyouya continues to possess unflappable physical and mental fortitude. It all comes down to what’s in Nana’s pockets.
Was she able to toss Habu’s phone—and/or her poison needles—in the odd moment Kyouya didn’t have his eyes on her; say, when he first started reacting to the contact poison? With Michiru and five other classmates present for the search of her pockets, she’d better have, or it’s Game Over!