Immunity to toxins, subservient to a necromancer.
Ghouls are deceased individuals who have been brought back from the dead under the servitude of their master.
Ghouls are rarely seen in the BlazBlue universe and not much is known about them. They have only been seen twice - during the Ikaruga Civil War, and almost immediately following its conclusion. It is known that they cannot die even if they have been decapitated, and the only way to stop them is to either completely destroy them, or to take out the individual controlling them, usually a necromancer. Drugs and Arts can be used to resuscitate someone and bring them back, but they will not survive any trauma to their head if resuscitated this way. A soul is not required for someone to be reanimated.
As well as being durable to most weapons, poisons barely affect them. Only humans have been seen resurrected as ghouls, not beastkin or other species.
Chronologically, the first time they've been seen in the series was when the Ninth Squadron of the Novus Orbis Librarium were sent to investigate a laboratory where a Nox Nyctores was presumed to have been kept. The squadron, consisting of Honoka, Jin Kisaragi, Karenjina Parsett, and their captain, Grimwood Huster, encountered an innumerable amount of corpses that were resurrected when they decided to leave. They were taken out after their necromancer, actually a proxy puppet controlled through the sorcery of Relius Clover, was found and defeated. 
The next time they were seen was on the outskirts of the 13th Hierarchical City of Kagutsuchi. They were summoned by the 16th Fuzzy to fight Mai Natsume, Bell, Kajun Faycott, Shiori Kirihito, and Taro Sasaga'e.  Some were set alight by Kajun's chemicals but the majority of them were destroyed before they went on to help Ragna the Bloodedge fight off Fuzzy and a Bone Dragon. 
- BlazBlue Spiral Shift - Hero of the Frozen Blade, Chapter 3
- BlazBlue: Variable Heart, Chapter 8
- BlazBlue: Variable Heart, Chapter 9