Tara Raeken

Tara Raeken is a human appearing in a series of flashbacks and as a "ghost" in Season 5 and Season 6 of Teen Wolf. Tara has limited dialogue and appears physically in only three episodes, but she plays a major role in the character development of her brother Theo while her backstory provides important details on the Dread Doctors plans.

History
Tara's disappearance and death at age eight is ruled an accident. In reality, Theo, who was younger, allowed his sister to die of hypothermia, trapped with a broken leg in a freezing creek. She repeatedly called out to her brother for help, but he just stood and watched. (Strange Frequencies)

Lydia Martin learns via a vision that Theo facilitated Tara's death in order to take her heart for the Dread Doctors. They implanted her heart in Theo's body to make him a genetic chimera, which then allowed them to make him a supernatural chimera as well.(The Last Chimera)

After  Sebastien Valet  is defeated,  Kira Yukimura  uses her sword to open the earth. Tara appears from the hole and drags Theo back in with her. ( Apotheosis )

In Season 6, we see where Tara  dragged Theo. In some nightmare version of Beacon Hills Hospital, Tara oversees her brother's personal hell. She mercilessly rips her heart out of Theo's chest over and over again. ( Heartless )

Theo briefly remembers this experience when he enters the hospital later in the season. (Memory Found)