Cole enters Phoebe's life as a predator, as a demon named Belthazor. He uses knowledge from Phoebe that he then uses on the Charmed Ones. He makes many attempts to take out the Charmed Ones, but always fails. Just when he has them near the breaking point Phoebe comes to him and the two of them end up in bed. Slowly he starts to realize that he has fallen in love her. Especially after Phoebe is stripped of her powers and alone with him, Cole can't bring himself to kill her and even encourages her to reunite with her sisters. Unfortunately the rage spirit he sent after them saw his love for the witch and posses Cole to take out the Charmed Ones. The Charmed Ones are able to push the spirit out of Belthazor and get the piece of flesh they need to vanquish him. While the girls prepare the potion Cole kills the Triad only after they try to take him out. Phoebe learns shortly there after that Cole and Belthazor are one in the same. Even with this knowledge she can't seem to vanquish him after he professes his love for her and allows him to escape. When she finally admits her sisters about what she has done she assures them that Belthazor/Cole will never bother them again. Only to have Cole show up to inform her he wants to be good and he wants to be with her.
After fighting along side Prue and Phoebe's close encounter with death, Phoebe welcomes Cole into her life and her sisters accept him as a part of Phoebe's life. It is an on going struggle with Cole to stay on the right side, but Phoebe's love always seems to win over his dark side.
The struggle seems to be over when an woman strikes back at Cole for the death of her fiancée. The power stripping potion only destroys Belthazor leaving Cole human and an innocent. His new mortality, however, does not last long. In an attempt to help the Charmed Ones, Cole goes face to face with the source and not only destroys him, but inherits all of his powers. As hard as Cole tries to fight the new evil within, he finally begins to give in, so long as he can still be with Phoebe. He even tricks Phoebe into being married by a dark priest who is only ensuring Cole's claim to the throne of the underworld.
Once Cole learns Phoebe is pregnant and another man is looking to become the source he offers him the chance to take his power. But the Seer tricks Phoebe into killing this other man and taking her rightful place as the Queen of the underworld. However when comes to choosing between her sisters and Cole, Phoebe follows her heart and helps her sister vanquish her husband, who still professed his love to her as he vanquished.
Cole, however did not met with death, he was in a wasteland trying to avoid being devoured by the beast there. He kept sending messages to Phoebe who then saw him and told there was nothing she could do for him, but she would always love him. In the waste land he began to collect powers from other who were sent there and then devoured. He began strong enough to kill the beast. Cole soon gains enough power to escape the wasteland in just enough time to save Phoebe. As the FBI agent shot at Phoebe he switched their positions so the agent was killed by the very bullet he fired at Phoebe. Cole then once again professes his love to Phoebe before vanishing.
Cole and Phoebe now divorced are still in love with each other, but Phoebe has chosen to move on with her life. In the meantime Cole is trying to suppress his powers and prove he can be good to Phoebe. His powers however have made him very very strong and he is beginning to think that no matter what he does he will be bad. To ensure this does not happen he tries to attack the Charmed Ones so they will come after him. They of course do, but before the are able to vanquish him Phoebe realizes what he is trying to do and refuses to help him committee suicide. Unfortunately, Cole informs her it is too late for that and uses his powers to throw the vial at himself. It doesn't work and both the Charmed Ones and Cole realize that no one can destroy him and both are terrified. In Episode 100 After so many attempts to vanquish Cole the sisters finally vanquish him.
