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".
In "Broken Dolls", Roy Harper was 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 whole-heartedly 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.
In "Three Ghosts",
In "Blind Spot", Roy approaches Sin to show her the effects the Mirakuru. Upon hearing of a sleazy judge being around the area, Sin poses as a hooker to entice him. When the judge approaches him, Roy appears and begins to pummel him. Sin is able to calm Roy down and then calls 911 to get the judge to a hospital.
In "Time of Death", 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.
In "Seeing Red", Sin encounters Roy completely under the influence of the Mirakuru. Sin attempts 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 leaves Starling City, she and Sin share a hug.
in "Uprising", Sin witnesses Roy fight off Danny Brickwell's goons alongside a blonde vigilante who resembles the Canary, making her wonder if she was Sara. Later, Roy calls upon Sin, along with the rest of the Glades' residents to stand up to Brick. During the fighting, Sin spots the Canary and realizes that she's not Sara. Later, following Brick's defeat and the subsequent return of the Arrow, Sin informs 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". She had a sibling-like relationship with Sara and deeply cared for her. Their bond was so strong that she was able to figure out Laurel, who had taken the identity of The Canary was not Sara.
- In the DC comics, Sin is much younger; she is a child who was destined to become the next Lady Shiva, but she was adopted by Dinah Laurel Lance and Oliver Queen.