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Chorister Rezh Baphix was the last Chorister of Akhaten. Much like all Choristers, he was a servant of the mummy. He sang the Long Song along with Merry Gejelh while in the pyramid. The unending song ended with him when the Mummy awoke, after Merry stumbled on a verse of the song and quit singing.
When the Eleventh Doctor and Clara Oswald entered the pyramid to save him and Merry, he was frantically continuing the Long Song, hoping the Mummy would not awake. The Doctor informed him that the Mummy would wake up even if he kept singing, and the Long Song would have to end with him so he could escape harm. Baphix stopped singing and teleported from the pyramid to save himself.
This person's name was never spelled on-screen, allowing for closed captioning to determine proper spelling. Unfortunately, there was discrepancy amongst closed captioners. The BBC One DVD went with Chorister Asbethix, while previous captioning done for BBC One presumably had it as Rezh Baphix(on iPlayer).
Biography from the TARDIS Data Core article, licensed under CC-BY-SA