Arrow: 10 Best Characters Who Only Appeared In One Episode
6. Harley Quinn
The Arrowverse in its infancy rarely, if ever made overt references to well-known DC characters such as Superman and Batman. However, this changed in Arrow's second season with the introduction of the likes of the Flash and the Suicide Squad.
The latter's debut gave way to the live-action debut of Harley Quinn, nearly three years before Margot Robbie's Harleen arrived in the DCEU. Following a mission brief at the A.R.G.U.S. headquarters, John and Lyla were arguing outside the facility's holding cells due to the nature of the mission and their teammates.
They were interrupted by a woman who claimed she could help them before she was silenced by an unimpressed Amanda Waller.
Although her appearance basically amounted to a cameo, it was still a surprise to see the iconic villain/antihero for a moment, and the fact that Tara Strong (Harley's voice actor in countless animated offerings and video games) reprised her role was icing on the fanservice cake.