- Harley Quinn became more preoccupied by the Joker and his schemes which Catwoman and Poison Ivy strongly disliked.
- Catwoman sided with Batman increasingly more often which made her untrustworthy to Harley Quinn and Poison Ivy.
- Poison Ivy became more preoccupied with the welfare of her plants rather than humans so she increasingly distanced herself away from Harley Quinn and Catwoman.
Despite these differences, the Gotham City Sirens did still support each other as friends, particularly Harley and Ivy.
Arkham Origins Incident
During the events on Christmas Eve, Dr Harleen Quinzel was a trainee psychiatrist who interviewed the Joker for a psychological report. Selina Kyle was an unknown criminal during this time and Dr Pamela Lillian Isley was working for GothCorp.
Formation of the Gotham Sirens
After the events of Arkham Origins, Harley Quinn fell in love with the Joker an assumed her own criminal identity, and Poison Ivy became poisoned by Jason Woodrue leading to her transformation. Selina Kyle also themed her crimes around cats and named herself as the master thief, Catwoman. At some point, the three villains met and befriended each other, realising the success of their crimes if they worked together. This led to the inevitable formation of the female trio known as the Gotham City Sirens.
While Catwoman enjoyed the team-up with Harley and Ivy, she secretly felt the partnership was doomed to end. When this came to fruition, Harley and Ivy felt that Catwoman's relationship with Batman was the catalyst for the break-up.
Arkham Asylum Incident
During the Joker's takeover of Arkham Island, Harley Quinn met Poison Ivy in her cell in the Penitentiary. Ivy begged Harley to be let free, as she sensed her "babies" were in danger. Harley seemed reluctant, since Ivy was not on Joker's party list, but gave in due to their friendship. Ivy was set free and made her way to the Botanical Gardens. Harley was a while later defeated by Batman and locked away. Ivy was infected by TITAN, which caused her plants to destroy Arkham Island. Not much later, however, she was defeated by Batman, too, and sent back to her cell. Catwoman was not present on the island, presumably having escaped prior to the Joker's return to the island.
Before Arkham City
After the Arkham Asylum incident, Harley Quinn's attention was purely on treating the Joker so she had no contact with her fellow Sirens during this incident. She and her "Puddin'" escaped their cells and stole a boat and ended up, to their surprise, in Arkham City. They made the Steel Mill their hideout and acted out their schemes from there.
After surviving the effects of TITAN, an enraged Warden Sharp sent Poison Ivy to Arkham City. Catwoman, however, rescued her from the TYGER guards and dropped Ivy off at Gotham Park. Catwoman said that she would water Poison Ivy's plants, which she then failed to do. Shortly after, Ivy was apprehended by Batman and sent to the Arkham City mega-prison.
Catwoman later went to Sharp's office at the asylum to take Two-Face's pictures of the blueprints of Arkham City. When TYGER guards burst into the room, Catwoman was pulled to safety by Batman. However, when she noticed the card of Two-Face's camera was missing, she looked down and watched as Two-Face swallowed the card. Furiously, Catwoman jumped down and knocked Two-Face to the floor, before being surrounded by TYGER guards. She was then taken to Arkham City.
Arkham City Incident
During the events of Arkham City, Harley remained separated from the other Sirens as she was too busy looking after the terminally ill Joker.
Meanwhile, Catwoman decided to call in Ivy's help when she needed a way to enter the Confiscated Goods Vault with a "woman's touch". Upon meeting her, an angered Ivy tied Catwoman up in her plants, tempted to kill her for not looking after Ivy's plants. When she explained this to Catwoman, though, the latter proposed to get Ivy her plant if she could help her into the vault. Ivy reluctantly agreed and helped Catwoman into the vault.
Inside the vault, Catwoman came across Ivy's plant, though, she killed it, as revenge for Ivy tying her up in her plants. When she returned to talk to Ivy, she blamed the death of the plant on Strange.
After Arkham City Incident
After retrieving her stolen goods from Two-Face, Catwoman presumably escaped the mega-prison without getting caught. What happened to Ivy is unknown, though she likely escaped custody as well. Harley escaped custody and returned to the Steel Mill, plotting her revenge against Batman for supposedly killing the Joker.
Later, Harley Quinn broke Poison Ivy out of Bludhaven, possibly voluntarily as the two Sirens were still friends. Here, she was ambushed by Nightwing who Harley fought and was taken down by Poison Ivy's plants.
Arkham Knight Incident
When Scarecrow returned to Gotham City with a plan to destroy the Batman, he recruited some of the Dark Knight's most dangerous villains, including Harley. Ivy refused, having decided that she would now permanently distance herself from humanity. This angered Scarecrow and led her to be locked up inside a safe house in the Chinatown district of Gotham, where Batman saved her. He then brought her to GCPD Lockup. He later needed her help in ridding Gotham of Scarecrow's Fear Toxin. She aided him voluntarily and successfully saved Gotham, albeit with the cost of her own life.
Harley meanwhile attacked a quarantine zone Batman and Robin had set up to attempt to cure the "Joker Disease" that has affected 5 people, due to them not getting blood transfusions to get Joker's blood out of them. While Harley managed to override Batman's security and free the "Joker clones", she and the clones were eventually caught by the Dynamic Duo, Harley being locked up in one of the quarantine cells.
Catwoman was not involved in the night's events, as she was inside the Pinkney Orphanage, being held captive by Riddler. She wore a special necklace, which required 9 keys to unlock and would cause her head to explode if she chose the wrong key. She was eventually rescued by Batman and aided him in taking down the Riddler, before heading off into the night.
After Arkham Knight Incident
When Batman and GCPD reclaimed control over the city, Harley was presumably arrested and taken into custody.
Catwoman likely evaded capture and returned to her thievery as way of living.
On the place where Ivy lost her life, a flower started growing - a small memorial for what she did for Gotham.
- Both Harley Quinn and Catwoman call Poison Ivy "Red", due to her hair colour.
- All three sirens named each other mostly by their criminal name (or part of it). The only exception to this is Catwoman, who Harley and Ivy usually call Selina.
- Never in the Arkham games have the Gotham Sirens worked together as a trio. However, in the Arkham Unhinged comic book series they have.
- Ironically, Ivy died saving many people, even though she said that she would destroy every so-called "meat sack" on the planet.
- Interestingly, all three members have at one point in time assisted the GCPD and Gotham during crisis. Harley Quinn assisted the GCPD during the Arkham Origins incident when performing a psychiatric evaluation on the Joker, Catwoman occasionally assisted Batman in his endeavors and Ivy assisted Batman in stopping Scarecrow from spreading his Fear Toxin across Gotham City.
- It is unknown how Harley and Catwoman reacted to finding out about Ivy's passing, although it can be guaranteed Harley broke down, having first lost the love of her life and then, when she was finally ready to move on, her best friend. Catwoman would probably keep her emotions under control, as she usually does, but would also be grieving, for she probably did consider Ivy one of her closest friends (despite their occasional arguments).