he'd moved like music, like someone dancing to a rhythm inside his head. And his face for a moment in the moonlight was the skull of an angel...
He reputedly kills various people at the Opera House, but also has been known to anonymously train an aspiring diva even if he thinks it is another diva wannabe. The ghost sends highly calligraphic notes to the the Opera House Owner directing him to have the ghost's protege in a particular role. Then sends notes warning of impending death and uses five exclamation marks!!!!!.
By the end of Maskerade, the Ghost is revealed to be two persons: Salzella and Walter Plinge, both opera-mad in their own ways. For Salzella, it ends in his operatic death, for Walter in a rebirth as the artistic director of the Opera House.
In Walter's case the Opera Ghost was a split personality born from Walter's years of being in the Opera plus the fact that his mindset made him "empty" and that emptiness was filled by Opera, resulting in the Opera Ghost being born.
This may be connected to Erik, The Phantom of the Opera as written about by Gaston Leroux (originally as a serialisation) and then turned into a musical.