Phoebe and Cole met when he came to town as the new DA, and Phoebe was in college. They were kinda thrown together in yet another unsolved case that involved the sisters. Cole thinks Phoebe is hiding something from him, but really it was he who was doing the hiding....he was Belthazor, one of the most powerful demons the Triad had.
He was sent to kill the Charmed ones, and instead fell in love with Phoebe, because he was half demon, half human....his mother a demon, his dad a human....mom killed dad...
He did make several attempts to kill them, just none that worked...and when Phoebe found out just who he was, she was going to vanquish him, but couldnt because she loved him so much...so she "pretended" to, and lied to everyone about it.
When she did finally tell her sisters, mad wouldnt and couldnt describe how they felt. Especially Prue, who had suspected he was a demon anyways. Even though each sister had fallen in love with a demon at one time or another, they didnt lie about it like Phoebe did.
To make an already long story short, they were married and then Cole was turned into the source and at first Phoebe was upset, but then decided to join him as Queen of the underworld, and she was pregnant. Although we found out it wasnt her baby, it was the seers. She finally came to her senses and helped her sisters vanquish him to the wasteland. Although he lived, since he was half human, he collected powers from other demmons who were vanquished and came back into Phoebe's life just as a judge was about to grant a divorce, from here, we can only guess where and what theyre going to do with them.