I won't name the author or the book, but I have since looked her up on Wikipedia and that solved another mystery. This story was engaging, but the writing was occasionally clunky. It turns out this author is one who cranks out three or four books a year. I'm not saying you can't write three or four good books in a year, but I am saying they are less likely to be polished if you do.
But one thing the author did well was character, especially because the main characters all had backgrounds that explained why they were the way they were. Sometimes main characters, in particular, suffer from what I call Athena Syndrome. In Greek Mythology, Athena burst forth from her father Zeus' head fully formed and armed to the teeth. That might work for a goddess, but every adult who ever lived started out as a child, and whatever that child's life was like exercised a profound influence on the adult he or she became.
I agree that a book has to have a plot to be readable, but if it doesn't have characters who are not only memorable but who come from somewhere (and not in the geographic sense), then that book is unlikely to make an impact on the reader. Real people have parents, sometimes siblings, and often extended family, too. Even if your protagonist is an orphan (the bane of YA fiction!), he or she needs to have some memory of childhood. Even if it's not actually in the story, the writer needs to know what the protagonist's childhood was like.
Plot might be the bones, but character is the flesh of the story. It's fine to write lean prose, but characterization is muscle that shouldn't be lost.