The Birth: Miles Warren was a brilliant scientist and professor at ESU. Miles had haboured a crush on Peter Perker's late girl friend; Gwen Stacy. When Gwen was killed by the Green Goblin, Miles Warren blamed Spider-Man for her death and went off the deep end. When his attempt to kill Spidey by sending the Punisher after him didn't work, he tried to clone Peter Parker and Gwen Stacy. At about this time Warren donned the identity of the Jackal to personally do damage on Peter Parker.
His first successful attempt at cloning was the creation of Kaine - a Peter Parker clone. But since his cloning technique was not yet perfected, Kaine slowly degenerated and was left with scars covering his face.
While Kaine first made his presence known in Web of Spider-Man #119, his origin was detailed in Spider-Man #60.

The Alter Ego: Kaine has no other identity, though at one time he did masquerade as Peter Parker. Unfortunately for Kaine, the cloning precess was not only flawed in its inability to produce an exact physical facsimile but also in unstable psyche it rendered to the clone. Kaine turned out to be nothing like the real Peter Parker where the nature of the person was concerned. He has been a fugutive from the law almost from the time of his conception. Being a clone of Parker, he had his memories but was sad since that life could never be his.

The Legend: When Kaine started degenerating, the Jackal tried to kill him. But Kaine grew more stronger, eventually surpassing Spider-Man in physical strength. Kaine escaped from Jackal's clutches and lived by himself. Later, when Jackal created Spidercide (another Spidey clone), Spidercide joined forces with Kaine. Infact it was Kaine who named this Parker clone as Spidercide.
Shortly after May Parker passed away, Kaine framed Peter Parker for a murder. The cops arrested Parker on the basis that his "fingerprints" were found on the murder weapon. Peter was busted out of prison by Judas Traveller and then went "truth searching". It was during this time that Kaine revealed himself to Peter.
Kaine worked as an assasin for quite some time. Besides his incredible strength and agility, he is also able to "burn" similar degeneration marks into his victims. He is also armed with claws similar to those of Wolverine but much smaller.
Kaine somehow turned full with remorse and gave himself up to the police where he has been in custody ever since.