Like his counterpart, Prefect Scylla, Charyb is a Gizoid - a class of mimetic robots used by the Nocturnus as advanced soldiers. His name presumably relates in some way to the Twilight Cage's Sector Charybdis.
Charyb is distinguished from regular Gizoids by being blue as opposed to beige. When Sonic and friends land the Cyclone on the Nocturne at the start of Chapter 10, Charyb and Scylla mockingly inform the heroes that they hold the last two stolen Chaos Emeralds, before the two Prefects flee to their respective redoubts. Sonic and Tails's group pursue Charyb towards the northwest.