Harley Quinn and Poison Ivy have had a strong relationship both as friends and later as partners, and including their romantic involvement in Suicide Squad: Kill the Justice League makes sense and would greatly benefit the game's narrative. The friendship between Harley and Ivy started in Batman: The Animated Series in 1993, and the relationship between these two supervillains has grown over several decades spanning multiple forms of media.
Harley and Ivy's relationship has been supported in several comic storylines, the Harley Quinn television series, and Harley and Ivy were partners in Injustice 2. The Batman: Arkham series, however, has ignored the pair's deep connection and fated romance. There are two minor cutscenes where Harley and Ivy are shown as friends, but their relationship in the Arkhamverse has to this point been largely missing, and shallow where it's present.
Although Poison Ivy seemingly gave her life for Gotham in Batman: Arkham Knight, one of the trailers for Kill the Justice League shows plants coming to life and trapping the titular team of anti-heroes. This not only points to Poison Ivy being alive, but it indicates she will be included in Kill the Justice League. If she is in the game, Ivy's romance with Harley could be a pivotal plot point, as it wouldn't be the first time that Harley ruins or abandons a Suicide Squad mission for someone she loves.
How Harley & Ivy First Met Prior To Batman: Arkham & Suicide Squad
Harley Quinn and Poison Ivy first met in episode 47 of Batman: The Animated Series, "Harley and Ivy," and their relationship was initially a forced partnership. The pair meet while on separate missions to steal from the Gotham Museum, and after the alarms go off, they have to work together to escape from the police. This initial incident is somewhat similar to how Harley helps Ivy in the Batman: Arkham series. Once they're out, Ivy immediately questions Harley's relationship with the Joker.
Harley and Ivy quickly become best friends and start to bond through a mutual determination to take down the patriarchy. They commit crimes and live together for the length of the episode. Ivy tries to show Harley how abusive and toxic her relationship with the Joker was, and for most of the episode, she succeeds in keeping Harley away from him. However, as is the case in many of their adventures, it isn't long before Harley runs back to the Joker.
Harley & Ivy's Decades-Long Friendship
There were a few other episodes in Batman: The Animated Series where Harley and Ivy developed their friendship, including a Superman crossover episode where they teamed up with Livewire. After the initial meeting, Harley and Ivy's relationship grew much stronger. Before the New 52 comics, they had a mini-series called Batman: Harley & Ivy that ran in 2007. After that, Harley and Ivy, along with Catwoman, made up the Gotham City Sirens, and the comic series ran from 2009 to 2011. Ivy's romantic interest in Harley was first hinted at in the Gotham City Sirens series. However, at the end of the series, Harley betrays Ivy and chooses the Joker over her.
In most storylines, including the Batman: Arkham series, Harley either decides to leave or is forced out by the Joker, but she runs back to him at any opportunity she gets. Ivy spends a lot of time and energy trying to convince Harley that the Joker is no good for her, but despite repeated failures, in the 2013 New 52 Harley Quinn series, Harley and Ivy were finally featured together as being in an official open relationship.
Harley & Ivy's Romantic Relationship Would Enrich Suicide Squad: Kill The Justice League
The development of Harley and Ivy's relationship over several decades made the modern romance they share that much more satisfying for fans that shipped them from the beginning. After the 2013 introduction to their relationship, Harley and Iv's romance began to be represented in other forms of media. In the show titled Harley Quinn, Harley and Ivy have a somewhat strained romance until Ivy realizes she loves Harley and not Kite Man. Their romantic involvement was also explored in Injustice 2, where Harley and Ivy got married in an alternate universe.
Although Harley and Ivy's relationship has been official for a while in the comics, other forms of media have taken a while to catch up. The Batman: Arkham series showed their interactions as similar to the old friendship they had where the Joker came between them. Introducing their romance to the Batman: Arkham games in Kill the Justice League is the next step in developing these characters and catching up with what they've become in other forms of media.
Harley & Ivy Were Just Friends In The Batman: Arkham Series
In the Batman: Arkham series, Harley Quinn is still very much in love with the Joker. She works for him to help him take over Arkham Asylum and stays by his side when he becomes infected. Throughout Arkham City, Harley helps Joker run his gang and acts as his nurse while he waits for Batman to cure him. Arkham Knight shows Harley seeking revenge and trying to make a new Joker. The interactions between Harley and Ivy are few and far between in this particular series.
Poison Ivy is only concerned with her plants in Arkham Asylum and Arkham City. It isn't until Arkham Knight that she shows compassion for anything else. At the beginning of Arkham Knight, Harley releases Ivy. Then, Ivy explains to Batman Scarecrow's offer to take over Gotham and how she turned it down. Batman takes Ivy to the GCPD for her protection.
Ivy spends most of Arkham Knight locked up until Batman asks her to help him disperse an antidote for Scarecrow's toxin. After Scarecrow releases his fear toxin across Gotham, Ivy helps Batman disperse an antidote by making the ultimate sacrifice. Ivy saves Gotham before turning to spores in Batman's arms. However, Ivy's apparent death doesn't seem to have stuck, as she may make an appearance in Kill the Justice League.
As far as their relationship goes, Harley and Ivy interact in passing. In Arkham Asylum, Harley lets Ivy out against the Joker's wishes and Ivy blows her a kiss before leaving. There aren't any other interactions in Arkham Asylum or Arkham City between the couple. Harley releases Ivy from a cell again in Arkham Knight, but only under the orders of Scarecrow. These are the only two instances where Harley and Ivy are together in the Batman: Arkham series.
How Suicide Squad: Kill The Justice League Could Use Harley & Ivy
Including the romance between Harley and Ivy in Kill the Justice League is the natural progression for the series and these characters. The trailer shows a pod, similar to the ones in Arkham Asylum, bursting open and the Suicide Squad being captured by sentient plants. It could be that Poison Ivy is seeking Harley out, or that Harley may team up with her in some way. A storyline where the pair team up and betray the Suicide Squad wouldn't be far-fetched given Harley's history of doing the same for the Joker in both the live-action movie and in the New 52 comics.
Ivy may also be angry with Harley in Kill the Justice League, because Harley was devastated when the Joker died; but the Batman: Arkham series didn't show her grieving when her best friend, and potential lover, died in Arkham Knight. Ivy is a headstrong woman that wears her heart on her sleeve, so if she's angry with Harley, it will be apparent. Regardless of how the two are introduced to players, Harley and Ivy ending up as a couple in Suicide Squad: Kill the Justice League is the only logical outcome.