- "My folks think I'm dead, too. That, or they wish I'd never been born."
- —Sin to Sara Lance[src]
On May 10, 2008, Cindy went to a sports game with her father. He took a photo of her and kept it in his wallet. The following year, Cindy's father was shot down while flying a plane over Lian Yu, and soon died from wounds sustained in the crash, leaving Sin an orphan. However, before he died he asked Sara to find and look after Cindy. At some point Cindy chose to use the nickname "Sin".
Working with the Canary
After the Undertaking, Sin was attacked by a group of men attempting to rape her, but luckily she was saved by the Canary. Afterwards she became the female vigilante's confidant, leading to Roy Harper being pointed in her direction when looking for the Canary. When asked about her, she led him to the Canary's hideout and knocked him out. When he woke, she looked on as he was questioned by the Canary, but she let him go when The Canary told her he was harmless. 
Sometime later, Sin shared breakfast with Sara Lance, the alter ego of The Canary, and they both talked about their families. She later encountered Roy again and met Thea Queen, and she mentioned to him how Thea didn't know about him being The Arrow's sidekick. When "the Mayor" attacked the event they were at, Sin was wounded and was saved by Roy and Thea. As she was recuperating at a hospital, she was looked over by Roy and Thea. She woke up later and told Roy that, because he saved her, she would keep his secret. She commented on how Roy was an idiot, and Thea wholeheartedly agreed, leading to Sin admitting she had begun to like Thea.
Not long after this, Sin told Roy and Thea of her friend, Max Stanton, who had gone missing for a week. She asked Roy if he knew someone that could help, but Thea commented that Roy could, and the three of them joined in on a search.
A Mirakuru-infected Roy approached Sin to show her the effects the Mirakuru. Upon hearing of a sleazy judge being around the area, Sin posed as a hooker to entice him. When the judge approached her, Roy appeared and started to pummel him. Sin was able to calm Roy down and then called 911 to get the judge to a hospital.
After the Canary's identity was revealed to her friends and family, Sin attended a welcome home party for Sara Lance, and when she was introduced to her (the others not knowing of their friendship), Sin was quick to hug her, arousing suspicion among the others. Later when Sara had reconciled with her family, she reassured Sin that despite her having her big sister back, she would always consider Sin as a little sister.
Later on, Sin once again encountered Roy completely under the influence of the Mirakuru. Sin attempted to stop several people from attacking Roy, as they misinterpreted the situation. Roy then pushed Sin to the ground and she later told Sara about Roy, citing that he didn't seem like himself. Just as Sara was leaving Starling City, she and Sin shared a hug. 
Saving the Glades
During the uprising of the people of the Glades against gang control, Sin witnessed Roy fight off Danny Brickwell's goons alongside a blonde vigilante who resembled the Canary, making her wonder if she was Sara. Later, Roy called upon Sin, along with the rest of the Glades' residents to stand up to Brick. During the fighting, Sin spotted the Canary and realizes that she's not Sara. Later, following Brick's defeat and the subsequent return of the Arrow, Sin informed Captain Quentin Lance that the woman taking on the guise of the Canary is not Sara. 
Initially, like Roy, Sin resented the rich because she believes most of them are greedy, arrogant and selfish. Despite her initial dislike of Thea at first, after Thea saved her, Sin developed a newfound respect for her. Sin initially disliked Roy for associating with her but then developed a friendship with him, often referring to him as "Abercrombie".
Behind the scenes
- In DC comics, Sin was an adopted daughter of the Black Canary that was hunted by the White Canary. On Arrow, however, Sin is a friend of Sara Lance, who is both the Canary and White Canary later on.