User names within the Derby system are known as authorization identifiers. The authorization identifier represents the name of the user, if one has been provided in the connection request. The default schema for a user is equal to its authorization identifier. User names can be case-sensitive within the authentication system, but they are always case-insensitive within Derby's authorization system unless they are delimited. For more information, see the Java DB Developer's Guide.
CALL SYSCS_UTIL.SYSCS_SET_DATABASE_PROPERTY( 'derby.database.fullAccessUsers', 'Amber,FRED')