Zef'No is a Kressari trader whose ship secretly carries Cardassian weapons to the Bajoran extremist movement known as the Circle, a link uncovered when Odo stows away aboard his vessel.
Zef'No appears in The Circle, the second part of the trilogy that opens Deep Space Nine's second season. He is the captain of a Kressari merchant vessel that calls at the station; his people are known as dealers in botanical DNA and have no military of their own, which makes his ship an unlikely suspect.
Acting on a tip from Quark that the Kressari are supplying arms to the Circle, Sisko has the vessel inspected. Zef'No protests his innocence and the search finds nothing, but Odo slips aboard in disguise and stays with the ship after it leaves.
Odo watches Zef'No take delivery of weapons from the Cardassians, proving that Cardassia is secretly arming the Circle in order to drive the Federation from Bajor.