She is an amalgamation of multiple characters from the comics
Including Mia Dearden. The writers hint to this by making Dearden her middle name.
This blog post sums it up better than I could:
When Thea Queen was introduced, it immediately became clear to me that we would see the second version of Speedy, Mia Dearden. Further proof of this soon came when we learned that Thea's full name was Thea Dearden Queen. However, since then, we have seen her draw elements from various characters which include the likes of Cheshire, Mia Dearden, Artemis Crook (Young Justice Version), and Arrowette.