Zera, also known as Zeratul (depending of the versions) is an entity possessing Daniel. It is trapped in Daniel's body and can talk to him. As he can only escape Daniel's body upon Daniel's death, his primary objective is to lure Daniel to his demise.
Daniel meets Zera at the beginning of the game. Zera wants to possess Daniel and control his body but he fails and is trapped in Daniel's body.
Zera is a reference to Zeratul in Starcraft. Because of the copyrights, his name was changed from Zeratul to Zera in the Steam versions.