Zarathos is the former fallen angel who took possession of Johnny Blaze after the latter made the deal with Mephistopheles. He is the source of Johnny's power as the Ghost Rider. During his early career as Ghost Rider, Johnny christened Zarathos simply as the Rider.
Zarathos is an angel, the Spirit of Justice, created and sent by God to protect the innocent on Earth. However, he was tricked and captured by demons. Brought down to Hell, he was corrupted and tortured with visions of humanity's depravity until he went insane. His mission to protect the innocent was perverted into a lust to punish the guilty. He became the Spirit of Vengeance.
After Johnny defeats Mephistopheles in Ghost Rider: Spirit of Vengeance movie, Zarathos was freed from Hell's corruption and has returned to his true, angelic form. By channeling Zarathos' pure angelic power, Johnny was able to resurrect Danny Ketch. This change in Zarathos resulted in the Ghost Rider's fire changing from orange to blue.