During the Romulan Neutral Zone Incursion in 2266, Decius dispatched a coded message to the Romulan praetor, providing a status update. As a result of this unauthorized action, he was reduced two steps in rank – despite the fact that Decius had influential friends within the Empire – by the commander who correctly believed that they were being shadowed by an Earth vessel.
Later, in response to his commander's refusal to attack the USS Enterprise, Decius successfully manipulated him into launching an attack by requesting permission to destroy the enemy vessel himself. His commander, refusing to relinquish his own authority, did indeed attack, but soon found that the Enterprise was, in fact, only feigning disablement, and found himself overpowered by the Earth vessel. Decius and the other Romulan crewmembers were killed when the commander initiated a self-destruct, refusing to surrender to the Humans. (TOS: "Balance of Terror")
Decius was played by actor Lawrence Montaigne.
In the revised final draft script of "Balance of Terror", Decius was described as younger than both his commander and a centurion in the same episode. Also, a bracketed stage direction commented that he "wears his helmet at all times!"
This character technically has the distinction of being the first Romulan to ever appear in Star Trek, even though his ears are not visible in the first view of him, as he is shown wearing a helmet that covers them. The first Romulan seen with the ears was Decius' commander, played by Mark Lenard.
Decius was also the only Romulan in the episode who was given a name, thus making him the first named Romulan character. Indeed, he and Tal (in "The Enterprise Incident") were the only Romulan characters with names in Star Trek: The Original Series.