He doesn't appear physically in the story because of his suicide prior to the events, but his presence is still felt.
He had agents from the World Health Organization and the Consortium at his heels.
He drove himself underground beneath Florence because of the hatred he was garnering.
Zobrist contacted his employees through webcam, using the Dante death mask as his cover.
He said he had no fear of death and would end up like Jesus as a matyr and that he was only trying to save humanity, not kill it.
He named himself "the Shade" because of his suicidal views and his ability to remain invisible.
When the Consortium pursued him through Florence one morning, he got up to a church roof, then said they would never find his virus.
With that, he stepped off the roof to his death.