Clay was a colleague of Professor Washu Hakubi at the Galaxy Academy, and was highly jealous of her superior intelligence and renown. The two, at one point, competed for the Galaxy Academy's director's chair, but Washu won out (Clay would claim Washu sabotaged his chances by substituting a raunchy porn tape for his presentation, while Washu claims that it was because Doctor Clay was a "bastard"). As a result, Clay had to settle for the post of head of the graduate school. He eventually left his position, swearing vengeance on Washu and the whole universe.
At some point, he came into the employ of a mysterious and powerful extra-dimensional being known as Lady Tokimi. For the next 20,000 years, Doctor Clay wandered the galaxy, doing certain services for Tokimi such as moving entire galaxies, while also performing many criminal activities. One of his known crimes was stealing the Shunga, the Galaxy Police's newest battleship and attaching it onto his own immense vessel. In OVA 2, Tokimi allowed him to bring Washu to her, in exchange for the rule of a single galaxy.
To this end, Clay had his robot Zero kidnap and replace Washu's daughter, Ryoko, completely duplicating her appearance, powers, and even memories. However, Zero also did something she was supposed to be incapable of: she copied Ryoko's emotions for Tenchi Masaki, which led her to defy her creator.
Zero was unable to capture Washu, however Washu came to Clay herself to reclaim Ryoko. Clay's ambitions for Washu, as he explains, go beyond simple vengeance and deliverance to Tokimi. Although Zero was able to copy Ryoko's memories and emotions, she was unable to make a perfect copy of the gem that gives Ryoko her power. Clay was baffled as to why this gem was impossible to duplicate, even with his own technology. As he explains to Washu why he wants to explore the gem, "... I don't know anything about it! I don't even know if it really exists or not.." He then has a flashback of Zero discussing her interaction with Lady Tokimi: "Her very existence is impossible, even though she was present right there in front of us". It is here Clay begins to realize the similarities of Tokimi and the gems, perhaps even their true natures.
When the tables turn on Clay, he launches his spaceship's self-destruct sequence and escapes. The ship begins to destroy itself by forming a black hole, however Tenchi is able to dissolve it by forming the Light Hawk Wings and saving his friends. Clay witnesses this and is eager to share this experience to Tokimi, but is caught when the spaceship Ryo-Ohki emerges and stops him.
In the end, Clay was arrested by Galaxy Police detective Mihoshi Kuramitsu, and the dying Zero was assimilated with Ryoko by Washu. Sometime after his arrest, Tokimi's servant D3 modified the memories of both Clay and Zero (now merged with Ryoko) to conceal Tokimi's existence.