Dr. Lieter Greenbox was the inventor who created the Gemkat 6000 for Warden Cyrus Meece, as a way of speeding up the production of the gems that funded Megakat Maximum Security Prison. Because the device was flawed, it turned its operator Rex Shard into a crystalline cat with a deadly crystal touch.
After Warden Meece, Greenbox was Shard's next intended victim of revenge, but the Swat Kats kept Shard busy long enough for Greenbox to escape back to Gemkat Labs along with Callie Briggs after they'd retrieved the Gemkat 6000 from the mines. Greenbox attempted to rewire the device, but was interrupted before he could finish by Callie's news that Shard was coming right for them. They attempted to flee, but Shard grabbed Greenbox and turned him to crystal. This left the rewiring of the Gemkat 6000 up to Razor, and Greenbox was eventually turned back to normal along with everything else Shard had crystallized.
Sometime later, Greenbox moved to a new laboratory and began working on another invention, the micro-brain repair unit, which he hoped would become a revoloution in home appliances. After finally completing the device, he appeared on a special Spotlight on Science segment of Kat's Eye News with Tab Mouser, a segment that was caught by Mac Mange. The Metallikats had recently come away from a botched attempt at robbing the Megakat City Mint, Mac with a lost hand and Molly Mange in even worse shape, not even conscious. Realizing that the micro-brain repair unit was capable of fixing himself and his wife, Mac broke into Greenbox's lab and stole it.
In using the device, the Metallikats somehow contaminated its programming with their criminal personalities. Naming itself Zed, it began to rampage through Megakat City, assimilating all it could onto itself (including Mac Mange!). Joining forces with the Swat Kats and Molly Mange to stop Zed, Greenbox hastily built a special device capable of instantly changing Zed's programming. Unfortunately, while touring the interior of Zed's massive body, the inventor became so amazed at what his genius had created that he went insane, joined with his creation, and destroyed the reprogramming device. Greenbox ordered Zed to steal the Mega Beam from Puma-Dyne Labs and use it to destroy Megakat City.
Zed did so, but before he could fire the laser into the city, T-Bone broke the connection between Greenbox and his creation. The stress of the seperation caused Greenbox to lose consciousness, and he was carried to safety by the Swat Kats as Zed's massive body collapsed. Greenbox seemed to be okay afterward, so it is unknown if the vigilantes turned him over to the Enforcers or merely decided to keep his brief psychotic episode secret between the three of them.
According to Mark Lungo's Animato! article on the series, Greenbox was originally conceived by writer Lance Falk as the main villain of the episode Destructive Nature. In Falk's original story, Greenbox was an insane half cat, half plant botanist who created the plantimals and used them to take over the Megakat Tower. However, a Hanna-Barbera executive suggested they re-use Dr. Viper for the episode. Taking his advice, Falk rewrote Destructive Nature into its current form.