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.
She Overcame Addiction
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.
She Learned Who Her Dad Was
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.
She Learned To Fight
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.
She Learned Oliver's Secret
After years of keeping his identity a secret from her, in season 3, Oliver finally revealed to his sister that he was the Green Arrow by taking her to the hidden room of the Verdant and showing her the Arrowcave.
He initially expected her to be angry after learning that was the reason he had been so distant since he returned from Lian Yu but to Oliver's surprise, Thea understood the importance of his role in protecting the city and was accepting of him carrying out his mission.
She Died And Came Back
Shortly after learning that Roy was "killed" in prison after getting arrested for revealing himself as "the Arrow," Thea is attacked at her loft by Ra's Al Ghul. Despite her training, Thea is unable to keep up with him, and the fight ends with Ra's slamming her through her glass table and stabbing her in the chest with a sword.
After Oliver comes to discover her, he along with the rest of the team travel to Nanda Parbat to request the use of the Lazarus pit to revive her. After coming back, Thea struggled with the bloodlust that came as a result of using the pit and her body started deteriorating since she refused to kill but was she ultimately given the cure by Nyssa Al Ghul.
She Became A Vigilante
Using the training she received from Malcolm, Thea eventually decided to follow in her brother's footsteps and it wasn't long before she became a vigilante herself. In season 4, Thea officially became a member of the team and began looking after Star City along with Diggle and Laurel after Oliver and Felicity left to go attempt and live a normal life.
She repurposed the arsenal suit that used to belong to Roy which he had given her before retiring and adopted "Speedy," the nickname Oliver had called her since she was a kid, as her codename.
Her Appearance Changed
Thea also went through several physical changes throughout the series during different points in her life. Between seasons 2 and 3, Thea cut her hair into a short bob while she was living in Corto Maltese with her father. While she maintained that look for most of the series, she began growing her hair back out during her time away from Star City.
Later, when she reconnected with Oliver in season 8, she also had a new scar on her cheek which she later revealed to Oliver were given to her by Athena, the leader of the Thanatos Guild.
She Got Engaged
Thea first met Roy Harper, who was a criminal at the time, when he attempted to steal her purse when she was in the Glades. She dropped the charges against him and later offered Roy a job at Oliver's club after he saved her from some attackers and the two started a romantic relationship. Their relationship was halted when Roy was forced to leave Star City after posing as the Arrow to keep Oliver from going to prison.
After visiting him, Roy broke up with her so she wouldn't live her life on the run with him and gave her his old suit. They wouldn't reunite until years later when Roy captured by Ricardo Diaz and Thea goes to rescue him. In the season finale, while they were out on a mission to rescue William from his kidnapper, Roy asked her to marry him and she accepted.
She Met Her Adoptive Sister
While Thea did lose her brother, she gained a sister in the series finale. At Oliver's funeral, while she and her recently-resurrected mother, Moira, were standing by her father's gave, Emiko walks over and introduces herself.
Emiko was first introduced in season 7 and revealed to be the daughter of Robert Queen and a woman named Kazumi Adachi. Emiko originally started out as a villain but redeemed herself before dying in the original multiverse but luckily was brought back by Oliver.
She Founded The League Of Heroes
After the team discovers a map with the locations of the three remaining Lazarus pits that Malcolm had found, Thea, Roy, and Nyssa set out on a mission to destroy them. After successfully destroying the pits, she reunites with Oliver and Talia Al Ghul in Nanda Parbat and helps them with their plan to retrieve The Chronicles of Al-Fatih.
After the mission was complete, Thea suggests to Talia that they should rebrand the League of Assassins as the League of Heroes and Talia agrees.