Like his counterpart, Prefect Charyb, Scylla 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 Scylla.
Scylla is distinguished from regular Gizoids by being red as opposed to beige - and, according to Procurator Shade, by being psychotic as well. This indictment is convincingly backed up by Scylla's maniacal, impish laughter almost every sentence. 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. Knuckles and Shade's group pursue Scylla towards the northeast.