Cole first approached the Charmed Ones disguised as an Assistant District Attorney. Cole is half-human, half-demon and his demon half was hired by the Triad to destroy the Charmed Ones. But while he was trying to get close to Phoebe in order to kill the three sisters, he fell in love with her and ended up killing the Triad. He has been on the run from demonic bounty hunters ever since.

Understandably their relationship has encountered many obstacles. When Phoebe initally discovered that Cole was a demon, she faked his death and allowed him to escape. After he returned, she was bent upon staying away from him, but she realized how much she loved him and needed him. Cole then volunteered to go undercover in a demonic Brotherhood in order to stop their evil plans. While undercover, his old mentor lured him back to the dark side by destroying Phoebe's love for him. After Cole revealed to Phoebe that his mentor has used a spell to turn him evil, Phoebe's faith in Cole's good side was restored and she managed to reverse the spell. He then began helping the sisters in their mission to protect innocents and vanquish demons.

By accident, Cole's demonic powers were stripped from him and he became human. Before becoming human he had proposed to Phoebe, but she told him that she wasn't ready to get married yet. While he was human he felt very unsure about himself and his place in the world. He had been a demon for over one hundred years and he was not sure how to be a human. Cole also felt insecure about his relationship with Phoebe because of her wish to postpone their engagement. Phoebe finally accepted his marriage proposal, and after the sisters vanquished the Source (for the first time) they married. However, without Phoebe's knowledge Cole had been possessed by the magic of the Source when the Charmed Ones vanquished him. Cole tried to fight the Source and retain control of his actions, but the Source proved to be too strong for him and Cole succumbed. With the Seer's help, Cole tricked Phoebe into marrying him in a dark way. Because of this dark ceremony the son that Phoebe concieved was evil rather than good. While the Source managed to suppress Cole when he first took over Cole's body, after he learned that he was to become a father Cole's influence grew stronger and stronger and weakened the Source, allowing Phoebe to realize through a premonition that he had become evil again. When Phoebe discovered that Cole had turned, Cole decided that he was willing to give his powers as the Source to keep Phoebe and their son. However, the Seer had other plans. The Seer convinced Phoebe that she needed to listen to her unborn child and support her husband. So in the middle of a ceremony Cole was performing to give his powers to someone else. Phoebe, under the influence of the Seer, arrived and killed the demon that Cole was attempting to give the Source's powers to, ruining the ceremony. Cole was then coronated as the new Source with Phoebe at his side, his Queen of the Underworld. While she initially supported him as his Queen, Phoebe quickly realized that as much as she loved him she could not give into evil. With the help of her sisters she vanquished Cole.

When he died, Cole was caught in the wasteland where demons go when they are vanquished. While the Source's power had been devoured by the beast that guards the wasteland, Cole managed to hang on because of his soul. He continued to hang on to his and Phoebe's love, running from the beast in hopes that Phoebe would be able to rescue him. Cole tried to send psychic messages to Phoebe and she would hear his voice whispering in the wind or he would channel his messages through others. Phoebe used a spell to travel to him in the wasteland to tell him that he needed to move on. Initially he refused, but he grew tired of running from the beast and decided to succumb to the monster. Just as he was going to allow himself to be devoured, Cole accidentally stumbled upon the discovery that if he absorbed the powers of vanquished demons before the beast could devour them, he could receive the powers. Through this method Cole managed to pull himself out of the wasteland. He continues to pursue his relationship with Phoebe, even though she does not see much hope for their marriage.

After trying to win Phoebe back but to no avail, Cole became doubtful of whether he can ever regain Phoebe's trust. No matter what good deeds he performed, Phoebe could only see him as a demon. As Cole saw Phoebe moving on with her life, dating and risking her life to save a new boyfriend, Cole sunk deeper into a depression. He received an offer from supernatural beings that are neither good nor evil called avatars, who wished to make him one of them, but Cole turned them away. Eventually, he became so tormented by his own cursed fate, he tried to tempt the sisters into vanquishing him to end his misery, by using a darklighter to put Paige's and Leo's life in danger. When they finally did try to vanquish him, it did not work. Cole has become invincible.

Cole's behavior became more insane in his attempts to gain Phoebe back. He asked a demon to mummify Phoebe so that he might have plenty of time to win back her affection. He also elaborately plotted to gain control of the Nexus under the manor so that when he tapped into it the evil would spread to Phoebe and she would give into their love. In one final effort to win Phoebe back, Cole cast a spell which altered reality, making it so that the power of three had never been reconstituted. He was disappointed to find that in this other reality he was trapped in a loveless married with Phoebe, who only remained his wife to protect Piper from being killed. By altering reality in this way, Cole resumed his former identity of Belthazor, making him vulnerable to being vanquished. Because Paige also crossed realities, she was able to unite with her sisters to vanquish Cole once and for all and reversed the effects of his spell.