Sympathy for the character
As an author, I'm naturally interested in how this happens, what the author has done to alienate me from Characters A and B and defend Character C, when clearly I'm "supposed" to feel the other way around. Here's what I've identified so far:
--Character C is more passionate than either A or B. Both A and B have drifted along, with vague ambitions that they've never really pursued, doing a lot less than they're capable of. C has smaller ambitions (for which C is criticized by both of the other characters), but at least C has pursued those ambitions with zeal. The thing that A wants most, A has never even taken the slightest step to pursue (and somehow sees this as C's fault).
--C is sort of quirky and hapless and, early on, is placed in a vulnerable and quite funny situation (from which A is completely absent). This was the start of my sympathetic connection to C. Right after this scene, A does something deliberately mean-spirited toward C, which made me dislike A. I think this act by A is where my sympathies were most firmly channeled into the pro-C anti-A camp.
--Early in the book, Characters A and C have a difference of opinion over a subjective matter. The book immediately implies (and continues to suggest) that Character A is "right" and C's views are pathetic. But as a reader, I'm unconvinced. Since this is a matter of opinion, I don't see how either of them can be "right," or why C's opinion isn't just as valid as A's. This perceived unfairness toward C shored up my protectiveness toward C. And at one point, C has a chance to thwart A's expression of A's differing views, but instead enables A to find a wider audience for those views.
--There's a scene where A blames C for something that was under A's control, not C's control. After several pages of fuming at C, A only seems vaguely aware that maybe the true blame lies elsewhere.
--C does some things that benefit B, but B has only contempt for C.
--Both A and C deceive each other. C confesses immediately. A punishes C for C's deception, but continues in A's own deception.
Maybe I'm wrong and the author is planning a twist; maybe I will find out in the last third of the book that C is supposed to be one of the good guys, and not a buffoon after all. So far I think not. (But if that happens, then this author is a genius at properly manipulating my sympathies.) Overall, though, whichever way this book goes, this has been a useful exercise in allowing me to see what can make characters likable and unlikable. In this case, I see that when a character is treated meanly and unfairly by other characters, the so-called justification of "but that character deserves it for being really boring/nerdy/annoying" doesn't always work, and the unfairness may backfire, leading the reader to sympathize with the supposedly boring/nerdy/annoying one.