Don't we all wish this was easy? That we could simply punch our characters in the gut and make them cry, and the reader would feel badly for them? (Not that I'd punch a kid. I just used the pic for dramatic purposes.) Well, technically, it would be punching a kid, because we write about teenagers.
How do we make even the most unlikeable character vulnerable? What does it take for the reader to sympathize (<-- awkward word) with the character?
Here is my take on it:
The character needs to crack. And by that I mean: break their fucking shell. Don't most teenagers have that invisible wall surrounding them? You know--the one that parents can't pierce (and it reminds me of the movie Independence Day, the part where they think they can nuke a spaceship and THAT'LL TEACH THOSE BASTARDS! and then they realize the nuclear bomb didn't do shit to the aliens' hidden, vaporizing shield).
And what did Will Smith and Jeff Goldblum have to do to finally take it down?
Well, nothing. They sent the drunk guy in to destroy it.
BUT the drunk took out the aliens from the center. As with your characters, you have to get to the core to break the skin. If you learn what makes them tick, what their weaknesses are, then you can take them down.
Let's say, for example purposes, we have Janey Jane as MC1 (main character numero uno), and Johnny Boy as MC2. Johnny's evil. Like, youdontwanttomesswiththisbecauseilluseablowtorchonyou evil. He's promised to take down Janey Jane's family and friends, in any way he can, if she doesn't keep her mouth shut about the murders that have happened lately in Anywhereville. One death is sad. But two? Two was suspicious. So Janey Jane got to the bottom of things. And she spotted Johnny Boy looming over one of the dead bodies--before the police arrived.
Janey learns to be smart. She has to if she wants her family and friends to stay alive.
So, she digs.
No, not literally. Just digs for information. Those murders? Yeah, turns out Johnny Boy was defending himself from his family. A family that sexually assaulted him all his life.
I bet you're sitting there thinking, "Aww, how sad." It is, isn't it?
But I'd also be willing to bet you thought Johnny was a monster at the beginning, too.
So, what changed? Well, it's different circumstances when someone has been led to do something rather than doing something on their own. It's also different when you can play on that emotions with writing--hurt, guilt, depression, pain, suffering. All these words immediately come to mind when I imagine someone living a horrible life.
What about when characters are evil just because? What if they have no motive? How can you show vulnerability then? Well, it can still come across in their actions, IMO. They might slip and help someone, or the way they act in certain situations may deter their ability to pretend they're hiding something sinister.
Whatever it is, there is still the ability to toy with the readers' reactions, and toy with your characters' emotions. How would you make a character more vulnerable?