Russol

From Memory Alpha, the free Star Trek reference.

Russol was a Cardassian Gul who was one of security chief Odo's most reliable informants. Russol was executed by Central Command in 2374, when the Cardassian Union joined the Dominion.

In early 2375 Weyoun 6 sent a coded message to Odo supposedly from Russol, asking to meet with him on a moon in Cardassian space. It was there that Odo found out that Russol was dead. (DS9: "Treachery, Faith and the Great River")

It is possible that Russol may have been Odo's shadowy informant (played by Joseph Ruskin) seen in "Improbable Cause".