Rippers are hired mercenaries who wield twin sickles. There are loyal to those who hire them. Rippers are Cleavers who have completed fifteen years of service. If a Cleaver does not wish to return to a normal life, they can choose to become Rippers. It is implied most Cleavers become Rippers.
The first Rippers were defective Cleavers. They had the same physical abilities of Cleavers, but lacked the mental component of mindlessly following orders. They wear black armour, rather than grey, which is identical to usual cleaver armour.
They are first seen in Death Bringer, where they are hired to guard for The Requiem Ball. Valkyrie asks if any of Rippers guarding the The Requiem Ball were going to help pursue Melancholia into the tunnels beneath the mansion. Skulduggery informs her that they are not Cleavers, only there to provide security for Gordon's Mansion. They corner the White Cleaver, but are defeated by him. This is after they effortlessly kill 5 necromancers.
They are seen again in The Maleficent Seven where they guard Johann Starke's house when he holds a party. Tanith infiltrates the party in order to steal the God Killer Dagger. After she gets it, she escapes on a boat with sabine, Sanguine and Springheeled Jack and a ripper intercepts them. It injures Tanith and Sanguine, pushes Sabine off the boat and puts Jack in a stranglehold. Jack's claws did nothing to injure the ripper because of its armour. Tanith then knocks him unconscious and, when faced with the dilemma of leaving him knocked out or taking him everywhere as a prisoner, Sanguine cuts his throat. It's highly likely that the ripper could have killed Sanguine, Sabine, and Jack had Tanith not been there.
Johann's rippers later apprehend Saracen Rue, Dexter Vex, Aurora Jane and (unsurprisingly) Wilhelm Scream. There was no talk of a fight so it's likely that a good amount of rippers surrounded them so quickly that, if they were to fight back there would be a slim chance of survival, even with Donegan Bane and Gracious O'Callahan having a clear shot with their sniper rifles.