Though she first appeared in a Dark Horse comic book early in 2003, Asajj Ventress' popularity skyrocketed upon her inclusion in Genndy Tartakovsky's Star Wars: Clone Wars animated series that began later that year, where she served Count Dooku as a Dark Acolyte. And you know that scar over Anakin's eye in Revenge of the Sith? According to Legends, that was her!
When the time came for LucasArts and Pandemic to add heroes to Battlefront II for Xbox-only DLC in Jan. 2006, Ventress was an easy choice. "We wanted a character where we had some control over the gameplay – a lesser-known character," says Eric Gewirtz, the game's creative director at Pandemic. "We wanted to save any marquee new characters for the next game."
Battlefront III never happened for Pandemic, but Ventress later became quite "marquee" in her own right, playing a major role in the CG-animated The Clone Wars. This cemented not only her appearance in several games based on the series to come, but also her indoctrination into proper Star Wars canon, which is what probably led to her being unlockable in Lego Star Wars: The Force Awakens. With EA's Battlefront II said span the entirety of the Star Wars canon – not to mention her inclusion in both handheld Battlefront games, Renegade Squadron (PSP, 2007) and Elite Squadron (PSP/NDS, 2009) – might we see her representing the dark side during the Clone Wars era?