in the novel it didn't really feel like that big of a deal. Don't get me wrong, it was hyped in the novel just as much, but the climax didn't feel worthy of foreshadowing.
I mean that's not true, any character that is omnipotent would beat any character that isn't so she automatically loses to the one above all for example.
That being said what is it then? Is it all those pictures of her feet?
a lot of things, Kaito's constant suffering because of his parents death (kind of the main theme of the story, every chapter of the web novel is told as if he's telling it to his parents), coming to terms with his feelings for Kuro and that he has a limited time to spend on that otherworld. Finally, to challenge and defeat shallow Vernal. The final Boss.
I mean that's not true, any character that is omnipotent would beat any character that isn't so she automatically loses to the one above all for example.
That being said what is it then? Is it all those pictures of her feet?
Shiro's primary ability is called "Epilogue," it brings about the end to any story no matter what. If the target is Omnipotent, they'll still be ended. If the target is Omniscient, they'll still be ended. If the target is immune to Epilogue, they'll still be ended. If they kill Shiro before she activates it, they'll still be ended. Shiro isn't omniscient nor is she omnipotent, but she is the ending to every story. Which is why she tried to create something for once (this world). It's also implied that Epilogue isn't even Shiro's strongest ability? And I'm paraphrasing this part, but I believe it was also explained that because of Epilogue, the reason killing her won't matter is because "as long as there is a story, there has to be an end, and that end is Shiro." Implying that Shiro will be the last living being in existence no matter what. So even The One Above All would meet his end to Shiro even if, by all other metrics, he blows Shiro out of the water.
For a non-spoiler answer: imagine that kid on the playground who says "nuh uh, I win because...", and that's Shiro's existence in a nutshell. Another, "not super accurate but the best I can think of" way to describe her without spoilers is "she's sentient plot armor." So if every being ever conceived of in reality or fiction all attacked her, she'd still win by nature of her existence, even the Author can't stop her because her attributes have already been set, so if the author tried to erase her, she'd still win.
Of course, this is never really used in this story and is more the readers going "wait... what the fuck did this author create!?" and following the logic to its ultimate conclusion. Shiro is quite literally unbeatable.
Edit: the vs Battles wiki put it more eloquently than I did:
The Epilogue: One of the true essences of the being called Shallow Vernal. As soon as she appears, the story has entered its final chapter. That’s why, no matter how hard her opponent tries, they met their ends. The beings that should have killed it, the beings that should have sealed it, the beings that should have created an ability to protect them from this being’s power, the beings that should have erased it from existing with the law of causality, and the beings that should have run from it. All of them were brought to their ends. More accurately this ability is the ability to always stand at the end in all conditions, with no countermeasures or ability holding any meaning to it. The Epilogue is the ability to close the book, end the story, thus no matter how one tries, compared to the Epilogue, they are mere characters in a book, of which the Epilogue is the reader. This ability has no limits on on its target's size, area of effect, or distance to be covered, and it can be activated regardless of Shiro's will, as the Epilogue has a will of it's own.
The Last Story: One of the true essences of the being called Shallow Vernal. She is also the final story, and thus will not disappear as long as other stories exist.