Jezek was one of the prisoners who escaped from SAR TOP Prison to Earth and took over a human form along with over 200 other escapees. Although what species he is isn't revealed, he is likely a Nodulian.
The reason why Jezek was imprisoned on SAR TOP prison was never revealed in the episode - nor what he was doing when he came to Earth.
Following his defeat of the Vardian called Zin (who had masterminded the escape) and his hiding of the dangerous Strada Brac, Cole/Daggon continued his hunt for the remaining fugitives. Seeking a way to capture all the remaining prisoners at once, Cole masterminded a possible way to accomplish this - but needed a test subject in order to ensure it would work. Soon enough, Cole was on his next hunt; Jezek been his new target.
Cole eventually caught up to Jezek in an airport, fighting with him in a storage area. Despite his best efforts, Jezek was outmatched by Cole and knocked down. With the fugitive temporarily brought down, Cole pulled out a device from which to perform his test - although Jezek openly voiced his dislike to been compared to a 'lab rat'. Performing the test, which was painless to Jezek, Cole quickly checked the results and found his plan to capture all the remaining prisoners could proceed. When Jezek questioned about whether Cole was going to take his life force, Cole immediately stated he was - now he had found out what he needed.
In a final attempt to escape, Jezek began using his superhuman reflexes to jump up and off the walls - hoping to potentially break free from Cole. However, Cole anticipated his moves and slid along the ground - catching Jezek in mid-jump and drawing out his life force. Following his capture of Jezek, Cole returned to the Watchfire and began his plan - explaining to Mel that his mission was nearly over.
Tracker (TV Series) - Episode 22: Remember When