From Degrassi Wiki
Heather Sinclair is known at Degrassi as the all-around queen bee. She's supposedly pretty, smart and popular, as well as mean. She's never actually shown on the show and therefore serves as a running gag. Paige mentions in an episode how only Heather Sinclair is meaner than she is. On The Click on the-n.com, there is a Degrassi Mini titled "A Day In The Life Of Heather Sinclair," in which Heather is still never seen, but we are shown the other characters' reactions to her mean comments instead. In season seven,
Holly J, Heather's younger sister, follows in her footsteps: pretty, popular, and mean. On another Degrassi Mini, "Queen Bees," Holly J and Paige have a face off for who is a better queen bee. When told that Holly J is Heather's younger sister, Paige responds "She's the devil's spawn!" Despite never being shown, she was almost shown in two season four episodes, "Neutron Dance", when she was taking a yoga class, but the camera focused on Paige and her arms covered her face and so only her hair was able to be seen, and in Time Stands Still, but she had got sick before the whack your brain contest. In the "A Day In The Life Of Heather Sinclair" mini, you hear her voice but you don't see her face.