Christy is the older sister of Billie Jenkins. She was kidnapped when she was 6 years old by Scabber Demons because she is the key to "ultimate power". The Scabber Demons kept Christy safe in the Underworld in preparation of the return of The Triad, a trio of powerful upper-level demons who were to guide Christy in the way of evil. Fifteen years later, Billie, her long lost sister, finds her and takes her to the Charmed Ones' Manor where Billie is staying at the moment. Billie begins teaching Christy how to control her powers. At one point Christy seems to be re-kidnapped by the same Scabber Demons sent by The Triad while in Billie's care, but Billie goes after her. While they are both down in the underworld, Billie convinces Christy to vanquish the demons. Together, with Christy's pyrokinetic powers and Billie's telekinetic powers, they kill the demons.
What Billie doesn't realize is that Christy is working with the Triad, the same trio of demons who sent Cole Turner/Belthazor after the Charmed Ones. It is originally believed that they are the major threat the sisters are supposed to face. This is not the case, as Leo Wyatt, Piper's husband, is not returned by the Angel of Destiny after two of them are vanquished. The remaining Triad member reminds Christy that Billie is the 'ultimate power,' and she must save her (in other words, turn her against the Charmed Ones). Christy succeeds in doing so.
Billie and Christy face the sisters in the manor with both sides empowered by the supernatural force called the The Hollow. Because they have both taken in The Hollow, their powers are evenly matched, since Billie and Christy both stole the Charmed powers from baby Wyatt. The power between shoots out in all directions, causing the manor to explode, killing Phoebe, Paige, and Christy. Billie and Piper are the only survivors. After the battle The Hollow returns to its containment.
Following this destruction, the Angel of Destiny brings back Leo, who stopped Piper from killing Billie. Piper goes into the past to fix the present. After leaving the destroyed manor, Billie returns to Magic School to seek help from Dumain, who suggests she Projects herself back into the past to warn Christy. However, during this he also attempts to trick her into going back and warning the now vanquished Triad, claiming only they could stop what was meant to be. Billie refuses this and goes back to her sister instead.
Meanwhile, Piper and Leo travel through time using Coop's (Phoebe's Cupid) ring. They find themselves walking in on Patty Halliwell and her husband Victor Bennett sharing an intimate moment around the time of Phoebe's conception. Piper is disgusted and shocked but not more so than her mother who is very confused about the two strange figures in her living room. They explain the situation and she agrees to help them. The set out to find Grams in the future. Grams is just as skeptical as her daughter was but also agrees to assist them. One last stop was made by the travelers. They find themselves in the manor with future versions of Piper and Leo. After returning to their own time they use the Power of Three (Patty, Penny and Piper) to send away the Hollow. At the same instant Bille returns from the past both Piper and Billie rejoin their present bodies. Bille takes hold of Christy and leaves. Billie returns with a treaty on her behalf. She then projects herself and the Charmed Ones into the past where they vanquish Dumain and The Triad just before they summoned the Hollow. Infuriated with rage, Christy hurls a ball of fire at Billie. The only thing Billie could do is reverse the ball back onto her sister. After Christy was hit with the flames Billie broke down. Billie and the Charmed Ones remain friends.
It could be assumed that due to her actions, Christy could be labeled as a warlock during her episodes on the show. But in fact, Christy is a witch turned evil through fifteen years of being raised by demons. She may have succumbed to evil, thanks to the manipulations of the Triad, but since she did not kill a witch for their powers, she is not a warlock, explaining why she was never explicitly mentioned as such.
Christy possesses two magical abilities: telepathy and firestarting.
Her telepathic power allows her to hear thoughts and project her own and is one of the more advanced versions of the power depicted in the show. Like The Source she can use it to communicate mentally between dimensions, maintaining constant contact with her demon masters as well as eavesdropping on The Charmed Ones thoughts from Magic School.
As a firestarter she can mentally cause objects and beings to burst into flame, and is shown to have enough control to reduce a demon to ash while not harming the surrounding area, and burn a rope tying her hands behind her back while blind folded, and not burn herself. She was also able to free herself from a crystal cage by focusing her fire power and killed a nearby demon in the resulting blast. In later episodes of the season, she displayed the ability to generate fireballs by focusing her power midair and on one occasion to propel her fireballs without the aid of her telekinetic sister Billie.