Most of the characters on Arrow managed to change throughout the show’s eight-season run but someone who arguably evolved the most was Thea Queen. Aside from her brother, Thea is the only character who ends up being vastly different from the person she was at the start of the series.
In the years following Oliver’s time on the island, Thea experienced a lot that helped her develop into the person she was by the series finale. While some of the changes are minor, most are completely life-altering and show just how much the character went on to grow after all that time.
When Oliver came back, he discovered that, due to his presumed death, Thea had become addicted to drugs. She began using a drug called Vertigo, which resulted in her being arrested for driving under the influence. She was originally going to have a much longer sentence but, thanks to Oliver, was put on probation and forced to do community service with Laurel.
Her own history and experience with addiction allowed her to help Quentin Lance deal with his alcoholism after the death of his daughter Laurel.
For most of her life, Thea was under the belief that Robert Queen was her father before Slade Wilson kidnaps her, revealing that her mother had an affair with Malcolm Merlyn and that he was her real father. This discovery drove a wedge between her, Moira, and Oliver since neither of them told her.
She and Malcolm would continue to have a rocky relationship with him riding the line between being a decent and horrible parent by doing things like forcing Thea to kill Sara Lance while drugged and eventually sacrificing himself to save her on Lian Yu.
After her mother’s death, Thea made the decision to leave Starling City with Malcolm and go to Corto Maltese. There, Malcolm began to train her using the same tactics that were done by the League of Assassins like teaching her how to have a higher pain tolerance by pouring boiling water over her hand.
While Thea was already a pretty skilled archer beforehand, Malcolm helped her refine her skills and trained her in several martial arts, showing her how to use various weapons, specifically swords, to the point where she was able to beat him on more than one occasion.