I'm not sure specifically as I haven't read this novel in detail. But generally, the main characters would be those who drive the plot forward, who face the main challenges and conflicts within the 'dark' context of the story.
The main theme could be about the shared human experience in difficult or dark times. It might explore how, regardless of our differences on the surface, we all face similar struggles and emotions when in the 'dark' situations of life.
No. Although both are from FromSoftware and share some gameplay elements like difficult combat, their stories are distinct. Sekiro is set in a fictionalized Japan with a focus on the story of a shinobi and his quest for revenge and redemption. Dark Souls has a more complex and interconnected lore spread across a dark, fantasy world with different kingdoms and a focus on the cycle of life, death, and the linking of the fire.
Definitely not. 'We All Fall Down' is a work of fiction. The story and its elements were made up to entertain and engage readers, not to represent actual happenings.