The first Birds of Prey casts doubt on that. Unlike most of Batman's rogues' gallery, Harley holds the odd distinction of not being born within the pages of DC Comics. Instead, Joker's signature companion was created for Batman: The Animated Series, FOX Kids' legendary adaptation of the Caped Crusader's adventures. Harley would prove so popular though that she eventually became canon everywhere, including the comics.
Quite a few movies featuring Batman have been made, and many of those also feature his nemesis The Joker. Yet, it took until 2016's Suicide Squad for Harley Quinn to finally make her way into the live-action realm. Perhaps that wait was for the best, however, as Margot Robbie's rendition of Harley received nearly universal praise from fans, despite the overall Suicide Squad movie proving much more divisive, and Jared Leto's Joker from the same project earning widespread complaints.
Suicide Squad already painted Harley as far more of an anti-hero than a villain, and 2020's Birds of Prey movie would cement that image, making her the lead protagonist and showing a much softer side of Mr. J's now former main squeeze. With that in mind, the idea that she participated in Joker's murder of Robin becomes increasingly hard to accept.
What We Know About Robin’s DCEU Death
Ben Affleck's Batman was introduced to the DCEU in 2016's Batman V Superman: Dawn of Justice, and that film revealed that Robin had been killed by The Joker at some point prior, with Bruce Wayne keeping Robin's suit and bike in the Batcave after as a reminder of the tragedy that happened on his watch. Director Zack Snyder has said that this occurred about a decade before Batman V Superman takes place, and costume designer Douglas J. Stewart later revealed that Joker set Robin's body on fire after killing him.
Batman would get a small measure of revenge by brutally beating Joker and punching his teeth out, leading to the famous metal teeth worn by Jared Leto's version of the Clown Prince of Crime. In Suicide Squad, a piece of onscreen text during Harley Quinn's character introduction lists her as an accomplice in Robin's murder. That piece of info completely screws up the DCEU timeline, as Joker already has the metal teeth in the flashbacks in which he and Harley meet. Timeline concerns aside though, there are deeper, character-based reasons why Harley's involvement in the sidekick's death just doesn't add up.
How Birds of Prey Changed The DCEU Harley Quinn
While Suicide Squad featured a Harley Quinn that was clearly not entirely evil, she still seemed committed to pleasing her twisted true love Joker, probably to the point of killing a superhero with him if he called for that. However, 2020's Birds of Prey and The Fantabulously Long Title Too Big for Theater Marquees flipped that idea on its head. Harley quickly befriends young Cassandra Cain in the film, and becomes fiercely protective of her, turning into an unlikely mother figure. She's entirely unwilling to allow harm to come to Cassandra, even at the possible cost of her own life, and the two head off to a new life together at the end.
Harley also ends up working well with her eventual Birds of Prey teammates, and seemingly forms genuine camaraderie with them. While some of this can be chalked up to character evolution following the dissolution of her relationship with Joker, some of her actions in Suicide Squad suggest that this Harley was always there, waiting to fully emerge when not directly influenced by the Clown Prince of Crime. Harley clearly has some level of moral standards that she holds herself to, something that definitely can't be said of Leto's Joker.
Why Harley Wouldn’t Have Killed Robin
With Harley's at least somewhat present sense of morals, and how quickly she grew attached to and became willing to put her life on the line for Cassandra in Birds of Prey, her readily taking part in Joker's killing of Robin doesn't make sense. It's never been a secret that Joker is more psychotic than Harley, and that their relationship is far from healthy, with Joker mentally and physically abusing her when he gets upset. She even seems to have her doubts briefly about their union in Suicide Squad. While Robin wouldn't have been a child like Cassandra when Joker killed him, it's unlikely Harley had a personal beef with him. She doesn't even seem to truly hate Batman. It's Joker that despises them, and uses grisly murder as a currency, not Harley.
While it's not quite clear exactly how Joker killed Robin, what Joker is seen doing to other characters in Suicide Squad, his reputation as a terror of Gotham City, and the fact that he set Grayson's body on fire leads one to believe that it was probably drawn out and torturous. Based on how Harley is presented in Birds of Prey, it just doesn't seem like she'd have that type of monstrous cruelty in her, especially toward a genuinely good person like Robin. Harley certainly isn't some innocent person with a squeaky clean past, or present for that matter. But she's definitely not Joker, and it's hard to imagine her laughing along as Joker destroys him.
How Harley Could Have Saved Robin
With it established that Harley Quinn likely didn't help Joker kill Robin, despite Suicide Squad's statement to the contrary, there's also a chance she may have helped him escape. Perhaps some other criminal was placed inside the Robin costume by Harley sometime after the capture, or an actually dead Robin was somehow able to be resurrected through supernatural means, such as Ra's al Ghul's Lazarus pit. Sure, that wouldn't quite explain why Robin never reconnected with Batman, but it's possible he can't, or just doesn't his former life at all.
Even if Robin were somehow alive though thanks to Harley Quinn, that was doubtfully the original intent of The Suicide Squad could use Harley's latest film appearance to do something similar.
