The Fifth Bureau was the personal intelligence agency of the Guarantor of the Alliance of Free Stars.
One of its agents, Markos, was a key figure in the coup d'etat that overthrew Walter III, the Elector of Kostroma. (RCN1)
Other members included:
There was significant amount of inter-service rivalry. The Fifth Bureau didn't have a high opinion of Alliance Fleet Intelligence.9:7
They also seemed less than admiring of Residential Services.8:11