The 14th, also referred to as "Pianist" and "Musician," and is the "Noah of Destruction" was a disowned member of the The Noah Family and the younger brother of Mana Walker. He was the only individual, besides the First Child of the Noah Family Road Kamelot and The Millennium Earl, who is able to control Noah's Ark. His true name was recently revealed to be 'Nea,' though it is not known if that is his human name or his Noah name.

35 years ago, he attempted to kill the Millennium Earl but failed, and was killed by the Earl (this accident is no longer remembered by any Noahs but the Earl and Road, who hasn't been reincarnated in 35 years). According to Cross Marian, after Nea's death, Mana lost his mind. However, Nea was able to pass his "will" to certain people (one of them being Cross), and Cross later revealed to Allen that he was implanted with Nea's memories to be the host for his revival, and "Allen" would be lost as Nea took over. When Nea's personality temporarily surfaced during the Noah's attack on the exorcist's headquarter, he revealed that he wanted to kill the Earl to become the new Earl.

Prior to his death, Nea was implied to be very close with the Millennium Earl, having apparently promised to "stay by his side no matter what happens." It's noted that in present days, mentions of Nea's betrayal is still painful to the Earl, and often causes him to cry. Even after Nea's revelation of his plan to usurp the Earl's position, he still confesses to the Order to "want to be by the 14th's side."
(Source: D.Gray-man Wikia)