Thea Queen (born 1995) is the sister of Oliver Queen, and daughter of Moira and the late Robert Queen.
History
Thea (it is insinuated) idolized Oliver as a child. They used to be close and according to Oliver, she used to chase after him when she was younger; that's why he give her the nickname Speedy. When her father and brother were both presumed dead, Thea felt alone — disconnected from the world.
Trivia
In "Pilot", Oliver uses her apparent old nickname, "Speedy", a reference to Green Arrow's sidekick in the comics who also happened to have a drug problem.