Be a sadist. No matter how sweet and innocent your leading characters, make awful things happen to them—in order that the reader may see what they are made of.
—Kurt Vonnegut, “Eight Rules for Writing Fiction”
A series introduces a character as sweet and lovable, more comic relief than anything, who likes nothing more than to pet little creatures. They make you adore them, root for them and love them.
Then the writers proceed to slowly torment them in front of your very eyes. They destroy everything important to them, kill everyone they love and make them suffer from horrible accidents, diseases and acts of violence, including but not limited to torture, rape or any other Fate Worse Than Death. They beat the character with one cruel stroke of fate after another until they are just an Empty Shell of their former cheerful, carefree self.
This technique is often used to build The Woobie in an attempt to enhance “adorability” points. Writers have to be careful or else The Cutie will become the universe’s Chew Toy.
Be careful about tormenting sweet little things—sometimes instead of breaking, or when there’s nothing left to break, they Freak Out. If they snap, you’d better hope your life insurance policy is up to date, especially if the cutie was a badass to begin with. If they break but refuse to show it, they could be a Type A Stepford Smiler.
Sometimes it can be Corrupt the Cutie, where the girl in question breaks it by herself. Frequently a part of a character crossing the Despair Event Horizon or the cause of a Heroic BSOD. Also frequently part of the backstory of the Broken Bird, and instrumental in the Freudian Excuse of a villain who Used to Be a Sweet Kid. Compare Fragile Flower and Wide-Eyed Idealist.
On a more constructive note, sometimes breaking the cutie can result in a cute but weak character Taking a Level in Badass as they confront their tormentors and become more assertive. When the cutie refuses to break, they might become an iron or stoic Woobie, a Determinator, or a Plucky Girl. If they are simply unbreakable to begin with, they are probably a Pollyanna.
Contrast Break the Haughty, where bad things happen to an arrogant person (who had it coming), Break the Badass (when a super-strong person is put through the gutter), or the even worse variation Kill the Cutie.