Before they sneak into the temple where Tadatsuru is keeping the girls, Koyomi and Ononoki are met by Oshino Ouji, who reminds him what her ‘uncle’ said: “All you can do is save yourself on your own”…even though Koyomi following through on that would mean he’d be open for extermination.
Ononoki surmises out of earshot that Ouji is the mastermind and ‘final boss’ that requested Tadatsuru’s extermination services. I’m not sure what to make of that since I’m still a bit fuzzy on who or what Ouji is, but in response, in the finale, Ononoki makes clear who and what she is, in spite of herself.
Taking what Ouji said about ‘keeping things proper’, Ononoki lists all of the things she is that make her unproper: apparition, shikigami, corpse, tsukumo-gami. She also, seemingly intentionally, deepens the significance of what could have been a simple matter of saving three girls from a hermit without being detecting, because while the girls will be safe, the basic problem of Tadatsuru being after him would remain.
She also tells the story of how she came to be: Kagenui, Kaiki, Oshino and Tadatsuru all collaborated on her production, which was “something like a Summer research project by bored college students”.
The trouble came afterward when Tadatsuru and Kagenui fought over who would have ownership over her. Kagenui won, because Ononoki chose her. Ononoki can therefore say, and be technically correct, that she was the one who causes a rift between the two, even if she was merely the straw that broke the camel’s back.
Ononoki mentions this heretofore untold story because she wants Koyomi to know that he can offer her to Tadatsuru in exchange for the girls. In other words, she’d give up her life to save him from spending his.
Koyomi reacts to what he deems a stupid offer by doing something stupid: flipping Ononoki’s skirt…then holding it in a flipped-up state.
The message he means to convey with his sexual harassment is that Koyomi doesn’t value his own life above hers, even if she’d be fine with him doing so. Ononoki concedes, then offers to kill Tadatsuru herself. He balks at that too, worried she’d lose whatever humanity she had gained in the time he’s known her. She even suggests he could get away with the girls by giving Tadatsuru Shinobu, knowing Koyomi wouldn’t go for that either.
Ultimately, she makes Koyomi let go of her skirt, and she agrees to go by his original plan where he acts as a decoy and stalls Tadatsuru while she swings around back and rescues the girls, leaving him on his own. It’s a plan we see Ononoki will unilaterally tweak once in motion; again asserting her humanity.
Tadatsuru himself…well, he’s a bit underwhelming, aside from being an origami folder par excellence. He seems impatient with Koyomi’s banter, but he’s also in no hurry to kill him.
In fact, Tadatsuru simply sits there above an offertory box as he and Koyomi chat, giving Ononoki the time she needs to sneak up behind him and cast Unlimited Rulebook at point-blank range, killing him. It’s something he almost expected, even requesting she do it with “human compassion” and deliver her catchphrase, “I said, with a posed look.”
So Ononoki ends up a killer, albeit one who acted on her own, against his wishes. She also quite likely saved his life and that of the girls, and Koyomi and Shinobu didn’t have to use any power. But it occurs to Koyomi that, all along, this was meant to drive a wedge between him and Ononoki.
After the girls are safe and sound, Koyomi pays a visit to Senjougahara, who reminds him with a chocolate to the mouth that it’s Valentine’s Day. He informs her of his present state, and she essentially shrugs it off: as long as she can see him with her eyes (and he does see himself reflected in them), why should he care about being seen in mirrors? If anything comes of it, he won’t have to deal with it alone.
Back home, Tsukimonogatari exploits one last sequence of Koyomi walking in on his half-naked sisters, who ‘coincidentally’ won Ononoki on the crane game earlier that day. To counter the objective of creating a fissure in Ononoki and Koyomi’s relationship, Gaen and Kagenui decided the best thing to do would be for Ononoki to move in with Koyomi and become even closer and ‘more intimate’ until the town stabilizes.
This, of course, creates an entirely different kind of tension, which may not be as serious as losing one’s humanity or being targeted for extermination by specialists, but a tension all the same: that of yet another girl in Koyomi’s life, competing for his time and affections, when he already has plenty.
But to put it another way, Ononoki is one more girl to protect him so he won’t have to vamp up…and the only one with UNLIMITED RULEBOOK!