It is revealed that Allen is the host chosen for the 14th Noah's revival, and was implanted with his memories at some point in his childhood while living with Mana, the 14th's older brother. The memories of the 14th Noah are said to gradually eat away at him until, eventually, nothing will remain of Allen's personality and he will be reborn as the 14th Noah. As a direct result, he has begun to see the shadowy silhouette of the 14th looming over his own reflection. Included among these memories is the power of the Musician, which allowed Allen to enter the 14th Noah's room and assume control of the Ark by playing the white piano within. In doing so, he halted the download of the Akuma egg plant, reconstructed the destroyed Ark and returned those who had previously been lost in its collapse, ultimately claiming it to be used as a tool for the Black Order. This power also lets any wish he wants to be granted, explaining how Kanda, Krory, Lavi and Chaoji come back from a lost dimension--despite Cross saying they were dead.
Cross states that the 14th chose Allen as he was running away with Mana and did not want to let this chance slip by. He also states that he knew both the 14th and Mana, and that the 14th told Cross to take care of Mana and if he died, the 14th would one day return to him (Mana). Another thing he states is that, Mana sort of lost it after the 14th died, leaving Allen to believe he may have not loved him at all - but simply the memories of his brother. Cross also told Allen he would have to kill someone he loved when he became the 14th. Cross ends his meeting with Allen by telling him to listen to the 14th and that there is "another side to this war".
Despite the prediction of his fate, Allen has vows to keep fighting no matter what, that he will not simply give in to the 14th's personality. The Order allows him to continue his duties under scrutiny, but puts restrictions on his Musician abilities. In addition, he is closely shadowed by Howard Link of the Order's special operations unit, CROW who is investigating and reporting his every move.
However, in later chapters Earl reveals that Allen isn't the player or the host of the memories that the 14th left behind, but he is the 14th himself. It is revealed after the 14th connected to Earl while Allen was fighting with the Level 4 Akuma during the Timothy Arc. It hasn't been revealed how the 14th is Allen, since it was disrupted by 14th reappearing and declaring that he will kill the Earl and take his title.