Nardo

 was a royal guard on Peladon. On the evening Pelleas became king, he had arranged to meet with his collegue, Frankis, near the ramparts of the Citadel of Peladon. He covertly witnessed at least some of a discussion that Frankis had with the new monarch. When the king amiably left after Frankis thoughtlessly mentioned his recently-murdered mother, Nardo popped out of hiding to tease Frankis about his social error. (AUDIO: The Bride of Peladon)

Behind the scenes
The character isn't mentioned in the credits on the Big Finish website, so it's unclear who played him.