SPOILER WARNING: This article contains major spoilers for "Star City Slayer," the latest episode of Arrow.
The mystery behind Katherine McNamara's character has finally been solved. In "Star City Slayer," the latest episode of Arrow, Mia revealed her true identity to William Queen. As it turns out, she is Mia Smoak, the daughter of Oliver Queen and Felicity Smoak.
In the present, Team Arrow spent most of the episode tracking down a serial killer who was sending them threatening notes. They soon discovered the culprit was Stanley, Oliver's friend from Slabside Prison. He was attempting to scare them off one-by-one, so that he was Oliver's only friend. By the time they realized that, though, he had already slashed Dinah's throat -- and he had already set his plan into motion. He managed to drug Oliver, Felicity and William so that they were conscious, but could not move, enabling him to explain how and what he was doing.
After Stanley attacked the Queen family in their home, they went to Star City Hospital to get checked out. Each of them had their blood drawn to make sure the drug he gave them had no adverse effects. However, that test revealed more than Felicity bargained for. She got an important phone call just after she said goodbye to William, who went to live with his grandparents. Though the episode didn't reveal what was said, she responded in shock and touched her hand to her belly, which strongly suggests she is pregnant.
Meanwhile, in the flash-forwards, Mia and Connor Hawke took the future Team Arrow hostage. Mia demanded passcodes to a surveillance program from William and Zoe so that she could find Felicity, who she insisted is alive. Both of them refused to give it up, buying Dinah and Roy enough time to fight their way free. A scuffle ensued, with Team Arrow overcoming Mia and Connor. William threatened to shoot Mia, who revealed that she is Oliver and Felicity's daughter, making her William's half-sister.
This, of course, explains Mia's grudge against vigilantes. She is Oliver's daughter but did not take his last name, suggesting they had a fallout or something happened to make her dislike him. Vigilantism, then, likely tore her family apart. This wouldn't be the first time for Oliver; after all, William decided to go live with his grandparents in the episode as well, since that was the only way he could ever live a normal life. It's yet to be seen how the circumstances surrounding William's decision will impact Oliver and Felicity's attitude towards this pregnancy, but it looks like it will have a lasting impression on Mia.
"I know that in talking to the writers and everything, they were trying to delay the inevitable figuring-it-out as much as possible, because they wanted to save the reveal for a bit later, in order to first establish the severity of the world and the twisted childhood that she had, which we’ll see more of as the season continues," McNamara explained to TVLine. "There’s a reason why Mia is the way she is, and there’s a reason she has such a resentment for vigilantes and for her own parents."
"As the season continues, we get more answers and it becomes very clear how dire the world has become in the future and the lengths that Oliver and Felicity had to go to protect their children. We don’t know what went wrong, but something went very wrong, very quickly for them to have to resort to these kind of measures," she added.
Airing Mondays at 8 pm ET/PT on The CW, Arrow stars Stephen Amell as Oliver Queen, David Ramsey as John Diggle, Emily Bett Rickards as Felicity Smoak, Rick Gonzalez as Wild Dog, Echo Kellum as Mr. Terrific, Juliana Harkavy as Black Canary, Colton Haynes as Arsenal, Kirk Acevedo as Diaz, and Katie Cassidy as Black Siren.