Romulus Sylvest was the son of Professor Sylvest and the twin brother of Remus Sylvest. Their father was worried their mathematical genius could be used to alter reality on a massive scale. His fears were borne out when Azmael abducted them on Mestor's orders and took them to Jaconda, where they were put to work solving the equations needed to explode Jaconda's sun and spread Gastropod eggs throughout the universe.
The plan was defeated by the Sixth Doctor, who returned the twins home. (TV: The Twin Dilemma)