Couple.
One girl, one boy; bound together by a number of things. Will they be more than just friends?
Before we go further, let's take a look at the meaning of the term "couple" defined by American Heritage of the English Dictionary, quoted by The Free Dictionary.
couple
n 1 Two items of the same kind; a pair. 2 Something that joins or connects two things together; a link. 3 (used with a sing. or pl. verb) a. Two people united, as by betrothal or marriage. b. Two people together
To avoid misconception and hard feelings, let me tell you that the term "Couple" I'm using in this site is closest to the first and second definition. Hence the underlined keyword: the same kind and connection. While we know that Allen/Lenalee as lovers have yet to happen as far as the story goes, but we can't deny that they are connected in many ways, and that they have some similarities.
However, as a big shipper of these two, I think it's also my job to share my thoughts on the possibilities and impossibilities of Allen and Lenalee to become lovers. Among other pairings favoured by the fans, I'd say that Allen/Lenalee is less popular. Probably not the least, but less, indeed. At least in the internets. As we see that the most of active internet inhabitants are female, and most of these females are fujoshi (yaoi / shounen-ai fangirls), so their popular choices are probably either Kanda/Allen or Allen/Lavi. Otherwise, they'll go for either Lavi/Lenalee or Kanda/Lenalee.
Here, I'll present you my take regarding Allen and Lenalee's potential relationship, based on the given facts. Some of these don't come from the manga, but still, they can be considered as evidences.