Mabel Stillman

You may be looking for her sisters, Margo Stillman or Mitzy Stillman...

Mabel Stillman was one of the three Stillman Sisters, during Season 6 they each stole the Charmed ones' powers and identities.

Since Mabel was the oldest sister, her identity target was Piper.

After Chris found out they were impostors, he turned the sisters against each other thus broke their bond along with their spell.

After the Charmed Ones got their powers back, Mabel and her sisters' magic was bound and were eventually sent to jail using the evidence of a man Mabel killed across the street earlier.

Original Powers
As a witch, Mabel has the powers of shapeshifting and conjuration.

Aside from that, she also possesses the basic abilties of a magical witch, such as scrying, spellcasting, and potion making.

Piper's Powers
After Mabel gained Piper's identity, she and her sisters used a spell in the Book of Shadows to steal their powers. Since Piper is the oldest, Mabel gained her powers of molecular immobilization and molecular combustion.

Though she had control over her new combustion power, she lacked the control of her freezing power until Piper tried to shoot her with a crossbow, causing her to panic and freeze the sisters.

These powers were returned after Chris broke their bond, and thus their spell.