prelude
I knew I've loved you before I met you. I have been waiting all my life.
What makes Allen and Lenalee (well, along with Cross Marian and the Millenium Earl, actually) differentiate from any other characters in D.Gray-man is that they were destined to meet way before the series even started. Doesn't make sense, you said? Well, sadly, you cannot deny that. We have evidence here, dear visitors. In fact, Hoshino Katsura had arranged their meeting in one of her older works, a one-shot manga entitled ZONE.
ZONE happens to be D.Gray-man's prototype, that's why it had similar characters and plot—except with simpler art and character designs. It has an exorcist, a sadistic lecturer of the said exorcist, a villain known as The Millenium Earl, and akumas.