Zero is a creation of Dr. Clay, and possesses the ability to mimic another being, including that individual's memories. Using this talent, she abducts and temporarily replaces Ryoko.
However, Zero was unable to duplicate Ryoko's gems, despite being endowed this power by Lady Tokimi. Perplexed, Clay has Zero use the originals to make a complete copy. While successful, not only does Zero gain Ryoko's powers but her feelings towards Tenchi as well. She defies Dr. Clay's orders when he ordered her to kill Tenchi when he feared that he was a liability in her mission. Afterwards, Zero was confronted by Washu, who had found out earlier that Zero was a duplicate of Ryoko when she found Dr. Clay's logo on "her" buttocks when they took a bath together. Zero was ready to face her and Ayeka to capture Washu, but it ended when Tenchi showed up during their confrontation and Zero chose to flee in tears, not wanting to face him.
When Zero confronts Clay to ask him to leave Tenchi alone, the doctor uses his ring to short circuit her, but somehow Zero survived. Later, after Dr. Clay was captured, Washu fused Zero with Ryoko, which allowed Ryoko to be more open in her feelings.
Like Dr. Clay, Zero also lost her memories of Lady Tokimi, because of her servant D3 blocking them off in an attempt to conceal Lady Tokimi's existence.