Detailed information (spoilers for v27 and v28)
Basics:
After Sizer's soul was stolen by Bass to put into the Daimaou sword, Vocal decided to gift all of her friends with a special present: bringing Sizer back to life! As a very strong level mazoku, Vocal has the power to basically compress all of his evil nature and sadistic glee - all of his ruling emotions, I suppose - into a small black sphere, and placed this within Sizer's soulless corpse in order to reanimate it. This creates Kuro Sizer.
In every respect Kuro Sizer is just a puppet: she has no mind or free will of her own, and her body is just controlled by a strong desire to kill, kill, kill as much as she can. At least Vocal will stop in his rampages to mock his victims, and occasionally kidnap them instead; Kuro Sizer unfortunately does not possess this 'merciful' side.
Her appearance is also warped by Vocal's gift of resurrection. Thankfully for Sizer her wings grow back, though they and her scythe are now jet black. She also takes on Vocal's style of clothing; leather, leather and more leather. XD With a side of snarling skull faces and spikes on her shoulder armour for good measure of course.
