Sinner's are the most rotten and vile of all creation. They are souls that are so warped and twisted by evil that when they die they do not become a hollow or go to Soul Society. They go straight to hell itself. In this hell all are condemned to endless torment for the rest of existance..however..there are those who have had the power to transcend this cycle..but not fully escape it.
These are the Sinner's. These are the strongest souls within hell that have found a way to escape hell. The only catch is they must wear a special mask over their face in order to leave. Should it be removed for any reason.. The gates of hell will appear and inrefusable drag that evil being back into it's depths.
Over the century since Ichigo's presence. Those strong enough to withstand hells torments have figured out how to strength that mask making it even harder to break..they found a way to make it in the same manner..as a shinigami's Zanpaktou.
The only real difference between a free Sinner and a Shinigami.. is a sinner has no release phase.. Which means he has access to 100% of his full might..all the time. Unlike shinigami..Sinner's are also not bound to a weapon to rely on their power. Their power is as unique and adaptable as a humans.
For the purpose of making this playable as a fun bleach race on this forum the mask has been strengthen to make it so it won't break unless the owner of the character wants it too or gets hit with so much undeniable power that it just makes no sense to have it remain intact.
We did this because lets face it.. a mask that a quincy arrow shattered? That would make it a one hit quit fight between a Sinner and any person in the bleach universe. So we did this to make them way more acceptable as playable people.