Let the Right One In (2008)

12-year-old Oskar is the constant target of bullies. He seemingly has no friends until he meets new neighbor Eli (pronounced "Ellie").  She is 12, "more or less".  As they become friends, she notices a wound on his face inflicted by the bullies. "Hit back," she says, "Hit harder than you dare. Then they'll leave you alone." He tells her it's three against one. "I can help."

He discovers that she is responsible for the town's murders and that something is not right about her. All of her windows are blocked, and she sleeps in the bathtub. She also happens to be a vampire. She points out that she only kills because she has to, while he is always plotting revenge against his bullies.

One of her victims survives, because she's kicked off before she can finish. The victim realizes she is getting strange symptoms and believes she must have caught it from the girl. In the hospital, she asks her doctor to open the blinds:

Look at that! That is some amazing special effects right there. Beautifully done. And since this is a vampire/human movie, I must say the famous words: Still A Better Love Story Than Twilight.