Rick's ex-girlfriend, the hive mind, is Unity. Unity can assimilate the minds of entire populations and control them as a single collective consciousness.
Rick and Unity reunite in the season-two episode “Auto Erotic Assimilation,” after Rick, Morty, and Summer respond to a distress signal. Rick and Unity resume their destructive relationship, spending their time indulging in chaotic parties and reckless behavior. Unity eventually breaks up with Rick after realizing that their relationship destabilizes her control over the assimilated population.
Unity is not simply one alien individual controlling others through technology. Unity is a hive-mind entity whose hosts are incorporated into a shared consciousness. The show depicts assimilation through a distinctive green substance passed between hosts, although the exact biology and limits of Unity's power are not fully explained.
Fans sometimes confuse Unity with Beta-Seven, another hive-mind character associated with her. Beta-Seven is an individual character and Unity's admirer, whereas Unity is Rick's former lover and the central hive mind featured in the episode. Unity later returns in “Air Force Wong.”