Nanclus
Nanclus was the Romulan ambassador to the Federation in 2293, where he was involved in a conspiracy to sabotage peace talks between the Federation and the Klingon Empire. 

When the Klingon Ambassador explained to the Federation President that James Kirk and Leonard McCoy were arrested according the articles of Interstellar Law, following the assassination of Klingon chancellor Gorkon, Vulcan Ambassador Sarek reluctantly agreed with his colleague's interpretation of the law. Sensing inevitable outcome of the situation, the Federation President inquired to Nanclus as to what the position of the Romulan government was on the issue. Nanclus wilily explained that "[i]n the absence of specific instructions from my government, I must concur with my colleagues." Not buying the answer, the President asked if he seriously believed Kirk assassinated the Chancellor, and Nanclus reluctantly responded, "Mr. President, I don't know what to believe."

In a later meeting, Nanclus lent his support to "Operation Retrieve," the proposal to infiltrate Klingon space and rescue the hostages, put forth by Nanclus' fellow conspirator, Colonel West.  He pushed his support by stating, "[t]hey ARE vulnerable. There'll never be a better time..." 

Later, when Spock finally discovered that Valeris was also a co-conspirator, he was able to determine from her that Nanclus was among her fellow conspirators.

Simultaneously, Nanclus was in attendance at the Khitomer Conference, where he sat with Vulcan Ambassador Sarek. As the conspirator's planned attempt to assassinate the Federation President was about to unfold, Nanclus glanced over to his visibly sweating co-conspirator, Vice Admiral Cartwright.

After the crew of the  learned of his involvement in the conspiracy, and successfully prevented the assassination of the Federation President at the conference, he was held at phaser-point by Doctor Leonard McCoy before being detained by Starfleet. ()

Appendices

Background information
Nanclus was played by Darryl Henriques. His jacket was later sold in the It's A Wrap! sale and auction, which was said to contain in its description "an interlocking design of Romulan-style hieroglyphs in various shades of blue, green, and yellow". 

Author David A. McIntee once proposed that Nanclus was actually the initiator of the conspiracy, having secretly put the idea that the Federation were the true threat to the Klingon Empire into General 's mind, due to Romulan concerns that the Klingons and Federation were about to form a pact. McIntee went on to characterize Nanclus as "almost certainly the true villain of the movie, and probably the most forgotten of the Trek villains." (Star Trek Magazine issue 149, p. 64)

Apocrypha
The novel  gives the Romulans a more prominent role in the conspiracy, revealing that Admiral Cartwright and General Chang were both influenced by telepaths under Romulan control. This suggests that Nanclus had a more active role than he appeared to in the film.

In the novel Cast No Shadow, "Nanclus" is said to be a Tal Shiar operative who used that name and the role of an ambassador as a cover. When the conspiracy was uncovered, "Nanclus" was recalled to Romulus, where the official report was that he was executed by disintegration. In reality, though "Nanclus" was given a different identity and assignment. In 2300, he was involved in a failed attempt to support Kriosian terrorists in rebelling against the Klingon Empire. After the plan failed, "Nanclus" and the other Tal Shiar chiefs were killed by a Klingon agent named Kaj.

External link
 

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Romulans
Ambassadors