The Cardboard Kingdom
(PNR/UF/Dys. orientated with a splash of anime)
Cardboard characters are a threat to all books, not just YA. What *is* quite unique to YA is the Cardboard Kingdom syndrome. This is where our heroine and love interest (and maybe villain/antagonist) are dropped into. What exists is the main couple (referred to as One True Pairing from here on). Their desires and their problems are the only ones that matter, and everyone else is simply there to serve. No one has a life outside said OTP. It's not just the world around them that suffers, but the OTP as well.
Because their relationship is the reason for the book's existence, the characters themselves sometimes don't get any characterization beyond one hobby, the one-off putdown (usually about their attractiveness) and sarcasm (and/or snarking at the Inferior Beings Surrounding Them). When we're talking about female POVs, typically the love interest is a robot. A Bad Boy robot, but a robot all the same. He's perfect - even his excusemedoyouhaveatrackrecordofabuse anger and possessiveness is portrayed as an Awesome Trait. He might have an ex, but she's just a slut. He might have a hobby, and it's there to make him more desirable as opposed to a hobby that he'd have because he likes it. That kind of hobby doesn't usually come to page because it would mean dedicating precious space that could be better used obsessing over our Love Interest. Worse yet is when we get male POVs, and the female love interest doesn't even have a non-hobby. She's just there to be liked.
When your main characters suffer from one dimensions, it can sometimes lead to more dynamic secondary/tertiary characters. But that only hurts the book, because the reader starts wishing they were reading from that person's perspective and it brings to the forefront that said book is really only about the OTP and nothing else.
Parents receive the brunt worst of all. There have been a lot of excuses, such as teens can't go on adventures with their parents hovering around their shoulders, it limits the story, etc, etc. Yet, wouldn't that create even more stakes? Wouldn't that make the character less one-dimensional and more relatable?
Everyone* knows what's like to sneak around your parents' back, the first time you lie to their faces for a reason other than your own skin, and the hazardous physical exercise of literally getting out of your house at night. For a television example, Cardcaptors: you've got your heroine who sneaks around town at night doing her job, and she has to balance her cheerleading, helping out friends, schoolwork, hiding a magical creature who eats like an elephant in her room, her job, and keeping from getting caught when she's leaving the house. Her dad is possibly the most understanding single parent of all time, but she'd still get in trouble for climbing out of her window at two in the morning on a school. And you know what? She manages it and she's only in the sixth grade.
I'd love to see more stories like this. I saw it in Possess, but not in much else. The heroine/hero who has to choose between friends/parental disapproval/possible arrest and the world? Who could resist that?
*If you have different background, ignore this.