It's so sad for Lavi to be watching the whole scene, unwilfully absorbing all the facts. However, at least Allen will not be alone and carrying the burden by himself. I have a feeling it will be Cross as the important person because after all, he is his master.
It slightly contradicts it when Allen realises that Cross had known about the "Fourteenth and Mana" all along and feels a little sense of betrayal. So ... maybe it's Lenalee after all.
Ahhh.. what a warm scene with Cross hugging Allen. It really touches you to think that was such an intense situation that Cross needed to hug Allen. I really wonder how Allen will turn into the Fourteenth.. If you think about it another way, maybe the Fourteenth's memories will consume Allen, turning Allen into the Fourteenth, and therefore Allen will turn to kill the Earl as the Earl was probably an important person to the Fourteenth.