A strong character in her own right, Harley Quinn is defined by strong relationships. When Paul Dini and Bruce Timm first conjured the character for Batman: The Animated Series in the 90s, Harley Quinn was designed to be a fun subversion of the hulking henchman trope. However, Harley's aesthetic and personality caused the character to slowly expand beyond her initial role into Joker's love interest. The only downside was that giving the Joker a tender relationship undercuts the menace he's supposed to generate. Consequently, Joker was depicted to be horrifically abusive to Harley, which became a problem as the character started to branch off into solo adventures.
Luckily, Dini, Timm, and the rest of the production team behind the DC animated universe provided Harley with an alternative paramour in Poison Ivy. Similar to how Joker defines one era of Harley Quinn, Ivy has come to personify Harley's more recent transition into one of DC's most popular characters. In the video linked above, HBO Max is looking to celebrate Harley and Ivy's iconic relationship with the trailer for a new Valentine's Day special.