A faithful friend or devoted follower:accompanied by three guides and his fidus Achates, Bruce set out...
‘You see, but you do not observe,’ he tells his dull fidus Achates; how many steps, for example, are there from the hall to the room where they are sitting?
This Isaac proved the most faithful of all his comrades, and stuck to him throughout the whole journey, a regular fidus Achates.
Origin
Latin, literally 'faithful Achates' (see Achates).