Ziiel is a criminal who escaped to Earth and took a human form during the mass escape from SAR TOP. Like fellow criminal Fenhill, he's never seen during the series and is only mentioned. The species he is isn't stated.
According to Nestov, Ziiel was a very 'anti-social' alien. He was sentenced to imprisonment on SAR TOP after murdering 43 Desserians before then going home and killing his mother-in-law (although, from what Nestov heard, some said his mother-in-law deserved it).
Thanks to the Vardian known as Zin, Ziiel was able to escape and take a human life force. However, some time later, Cole/Daggon was able to track him down and capture him. Before taking Ziiel's life force, however, Cole learned from Ziiel that Zin was "close to finding what he's searching for" - referring to whatever was hidden under the Watchfire. He later explained his capture of Ziiel - along with his message - to Mel and Nestov.
Tracker (TV Series) - Episode 20: Back into the Breach (Mentioned Only)