Owarimonogatari, Part 1: End Tale

Before we witness the series' climactic showdown in the third volume of the End Tale--each part of which forms its own cohesive whole--narrator Araragi wrestles with a crucial bit of history that had turned him into the loner we met at the very beginning, who opined that friendships only lowered his
intensity as a human.
What initiates his pilgrim's progress of a reckoning is his first encounter, at school, with the mysterious freshman Ogi Oshino, self-described niece of the equally enigmatic aberration expert Mm, and the book's opening chapter is a harrowing standalone novella of a who-dunit involving a locked room of sorts.
Our increasingly well-adjusted hero kept on beingdecent at one thing even when he was just hanging on, but this forte, an unlikely aptitude for math, of all things, becomes the focus of a cheating scandal and a web of recollections that forces him to come to terms with, what do you know, his capacity to connect to people.