"I am a loyal servant of the doctor." - Zero-Ryoko's thoughts after Dr. Clay's order to kill Tenchi.
Zero (零, Zero) is a robotic creation of Dr. Clay, and possesses the ability to mimic any being's appearance, including that individual's memories, emotions, and abilities. Using this talent, she abducts and temporarily replaces former space pirate Ryoko, in order to kidnap Professor Washu Hakubi.
Tenchi Muyo! Ryo-OhkiZero is first seen when Dr. Clay reports to Lady Tokimi. After the meeting, Zero indicates her fear of Tokimi (as she cannot quantify her existence). Dr. Clay suggests she needs to have the remains of her previous personality purged, then orders her to focus on their new assignment: abducting Washu without harming her. To achieve this, Dr. Clay sends Zero to replace Ryoko, infiltrate Washu's home, and eventually kidnap her in a moment she would be alone. On Earth, Zero catches Ryoko off-guard and becomes a perfect copy of her. Despite being endowed the power to duplicate everything by Lady Tokimi, Zero is unable to duplicate Ryoko's gems. Perplexed, Clay has Zero take the originals from Ryoko. Being an exact copy, Zero gains Ryoko's powers and her feelings towards Tenchi as well. Eventually Zero becomes infatuated with Tenchi, being unable to capture Washu. Clay, seeing this as liability to the mission, asks her to kill Tenchi. She defies Dr. Clay's orders. Shortly after, Zero is confronted by Washu, who had found out earlier that Zero was a duplicate of Ryoko due of noticing Clay's symbol on her buttocks, when the two were taking a bath together. Zero chases Washu believing that by capturing her, she won't have to kill Tenchi. However, Washu manages to avoid Zero by replacing herself with a doll version of her. Princess Ayeka Jurai, along with her guradians Azaka and Kamidake confront Zero. Zero uses her powers to push Azaka and Kamidake away and then threatens to kill Ayeka if she gets in her way. The noise and comotion causes Tenchi to wake up and witness Zero confronting Washu. Zero bursts into tears, and leaves. Washu, Tenchi, and Ayeka follow her to Clay's ship to rescue Ryoko where they find Detective Mihoshi Kuramitsu fighting some of Clay's robots (she was sent to investigate Clay's ship, which was actually a stolen warship belonging to the Galaxy Police).
Washu reaches Clay alone, and after a talk, Zero appears, confronting Clay, confused by her new feelings. She begs him to leave Tenchi alone, and Clay says he just needs to erase her memories to get rid of those feelings. Zero tells him that she wants to keep the memories and body so she could live as Ryoko. Hearing this, Clay uses a device on his ring to short-circuit Zero. Later, she regains her strength and frees Washu from Clay's grasp. Later on, after Dr. Clay was captured and Ryoko saved, Zero apologizes to Tenchi for lying to him, and says she wants to be called Ryoko forever. Washu fuses Zero and Ryoko into a single being, allowing Zero to live on within Ryoko, and allowing Ryoko to be more honest with her feelings.
Like Dr. Clay, Zero (in this case, Ryoko after being fused with Zero) also lost part of her memories of Lady Tokimi, due to D3's intervention. She would be later mentioned in episode 19, when it is revealed that Ryoko's knowledge was enhanced exponentially after the fusion.