Odo's Cardassian informant.
An informant or informer was a person in a government or organization that provided information to an external source. In 20th century Earth slang, an informant was known as a fink.
During the occupation of Bajor, Trentin Fala was an informant for the Bajoran resistance when she worked as a cleaner at a Cardassian records office in the Dahkur Province. (DS9: "The Darkness and the Light")
In 2366, Data described Haritath as his informant on Tau Cygna V. (TNG: "The Ensigns of Command")
Coutu's Parada rebels had several informants in the Parada government by 2370. One of them relayed the rumor that Miles O'Brien had been abducted and replaced by a replicant, possibly to kill their delegation at a peace conference on Deep Space 9. (DS9: "Whispers")
In 2371, Enabran Tain mused during the Battle of the Omarion Nebula that there must have been an informant for the battle to be so devastating for the Romulan-Cardassian fleet. The informant was in fact Lovok. (DS9: "The Die is Cast")
By 2372, according to Neelix, the Mokra Order had informants everywhere on their planet. (VOY: "Resistance")
In 2373, a member of Starfleet became an informant for the Orion Syndicate when he met Raimus on Risa. (DS9: "Honor Among Thieves")
The same year, Cing'ta was an informer for Starfleet from within the Maquis. (DS9: "For the Uniform")
Gul Russol had been Odo's most reliable Cardassian informant prior to his execution through Central Command in 2374. (DS9: "Treachery, Faith and the Great River")
When Benjamin Sisko approached Garak to steal secret data from Dominion Headquarters later that year, Sisko told him to use his contacts like a colleague from the Obsidian Order, an old friend or a reliable informant. (DS9: "In the Pale Moonlight")
Koval, the chairman of the Tal Shiar, passed information to Starfleet since the start of the Dominion War and may have worked even longer for Section 31. (DS9: "Inter Arma Enim Silent Leges")