The Phalanx are a group of people who are Number One's exclusive guards. They're ultimate loyalty is always to him though they may guard other people on his behalf.
It is not known exactly how they came about but they probably were commisioned after Number One has his body taken over by the A.I. Overlord.
In Escape Velocity they are assigned the position of guarding the Contessa as she becomes the new headmistress of H.I.V.E. after Dr. Nero is imprisoned by H.O.P.E. Though they guard her their loyalty is to Number One.
They dress in suits and wear glasses which give a sophisticated tactical heads-up on their surroundings, on their lapels they wear a pin-head with the face of a skull on it which is the only clue to their identity.
They do not refer to each other by name but by their number e.g. Phalanx One or Phalanx Five.