Pianist, also called "musician" and "noah of destruction," is a disowned member from the the noah family and mana walker's younger brother. he can control noahs ark, making him unique among family members except for road kamelot and the millennium earl. his true name was recently unveiled as nea, though it's uncertain whether that's his human or 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."