Zero-Shiki (零式（ゼロしき）, Zero Shiki) is an O-Part wooden boomerang that have the effect of double strength (increased power). It can be infused with any different power and to double it, for example Jio/Satan absorbed the lighting from the Lightning Axe and put it into the O-Part, thus increasing it's power 2 times. Although being wooden, it has great strength and is capable of cutting even through the hard giant worm, which a sword wasn't even able to scratch it. Jio often uses it as distraction, first throwing it at the opponent and if he dodge it, he follows with immediate kick and then hoping for surprise attack from the back, when the boomerang returns.
Jio found this O-Part when he was fighting along with Zero against the giant desert worm, and after hitting it with a giant rock, the worm spit it out along with various other items. The worm was eating anything and it often hunted at underground ruins that haven't been excavated yet, so presumably it ate the O-Part from such ruins. Jio used the O-Part as item to distract the worm, but was surprised that it was able to kill it. He also named the O-Part after his master, Zero.