Sizuka witnessed the fall of Earth when she was still a child. She could only watch as the Synd swept through, wiping out mass swaths of humanity at a time. Orphaned and full of rage, Sizuka was taken in by a group known as the Slag Hearts--Vais Resistance fighters who had been left behind on Earth. With their aid, she perfected the art of killing. Through her training she became removed from emotion--particularly empathy--but found it a small price to pay. She was too consumed with becoming the ultimate weapon and ridding Earth of the Synd for good.
When the Dredgion arrived on Earth, the Slag Hearts recognized their opportunity to finally gain the upper hand. Sizuka was sent out with a beacon to attract the Dredgion's attention and act as an emissary to this new force. Recognizing her incredible potential, the Dredgion put her through the tests to be made into a Master. Sizuka agreed to the honor, primarily interested in utilizing the Dredgion's additional resources to hunt down and eliminate more Synd targets than ever.