Tokyo Ghoul

Japan. Tokyo. A slew of ghouls have started appearing. Ghouls are former humans who, after being reincarnated, became cannibals and are incredibly strong. They feed on humans; they can't eat other food; it makes them sick. But ghouls also love coffee. In a Tokyo alley, there's a coffee shop called "Anteika." Peaceful ghouls work there, feeding on coffee rather than humans, so no one knows they work there, and even ordinary people come in. There's also a group called the CCG—they're like ghoul hunters—and they have their own Quinque (a Quinque is a weapon that appears after a ghoul dies; each ghoul has a different Quinque). Ghouls usually try to blend in with humans, so it's hard to tell who's a ghoul and who's a human.
Mei has a best friend, Ken Kaneki. One day after a date with the "girl," it turned out she was a ghoul, Rize Kamishiro, one of the most powerful ghouls. However, during their date, a building collapsed on her and Kaneki, killing her and requiring a transplant. Kaneki has since become a half-ghoul. But he hid this from his friends, afraid of being caught.