5. Illyria
Appeared in: Angel An ancient one who walked the earth long before the time of man, Illyria was born of Fred - literally - taking over her body and eating away at it from the inside. The death of a key character is never easy, but having the actress who played her remain, portraying the husk of the beloved Fred walking around with an ancient evil inside is something that only the most talented - and cruel - writers could possibly imagine. Illyria was a constant stab at the viewer's heart, while becoming a fascinating character in her own right, as she learned about the world around her, her place in it, and the new-found limits that existing within a human body (albeit with great strength) brings with it. Illyria saw humans as ants, even as she became more like one of them, and her relationship with Wesley especially, who had loved Fred, was both painful to watch yet riveting. Illyria was one of the boldest moves by Joss and his team of writers - the destruction of an existing character, the corruption of her image into something new, and ultimately, the birth of something just as interesting as before. Though not nearly as sweet and good-natured.