Thalia black was the long lost sister of Katherine pierce. Katherine didn’t know she had a sister, Thalia didn’t know she had a sister either. When Katherine got caught up with klaus Thalia was born while her sister was on the run. Thalia was five when klaus attacked her parents. She found a hiding spot so he didn’t see her but she heard everything. The screams, the tears, the pain. When klaus left Thalia came running over to her mom. Her mom’s last words were “find Katrina”. Thalia didn’t know who that was.