Zone.

Trust me, those two have met way before they were born.

Both Allen and Lenalee are characters that have already appeared in Hoshino's earlier work, entitled "ZONE". This one-shot manga appeared in Akamaru Jump in 2003. The character designs are strikingly similar with Allen and Lenalee.

While Allen's prototype, Robin, is a quite confusing character due to the gender-bending, Robin did show some interest and quite taken a liking to the past Lenalee. Robin, just like Allen, blushed when he (she?) saw Lenalee smiled. The restaurant owner even told him that he mustn't fall for her, since she was already engaged—but apparently the said man had already died. Robin also told Lenalee that she looked like an angel when she smiled.

While this point brought my hopes up, I kind of get the feeling that Hoshino wanted to give an impression that either Allen or Robin had some one-sided feeling towards Lenalee. Perhaps this was coming to my mind because in ZONE, Lenalee already had a fiance and Robin's gender is like... a female soul in a male's body. Hah. Also, by the end of this oneshot, Lenalee mentioned that she was going to find a new love. Would that new love happen to be someone named Allen Walker or not is a question I had in mind.