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My theory goes like this I believe that the scarecrow that appeared in one piece chapter 875 IS NOT PEDRO's SOUL, like everyone is saying I believe it's his best friend ZEPO.
Big mom put the life span she took from zepo and put it into the scare crow, and thanks to pudding flashback ability she's uses in chapter 876 gave zepo his memories back.
. With his memories back zepo will look for Pedro but will find job dying because of the explosion.zepo will give Pedro enough of hit his life span to come back to life only for little while longer.
Pedro and zepo will go to cacao island to help luffy and his allies escape and both will use their sulong transformation in the process.
But because of their weaken bodies and limited life force they can't control their transformation and start to go berserk.
But they both use they Will power to control their abilities enough to help the straw hats escape but at the cost of their lives, but they both fur smiling knowing their complete their goal of starting the dawn of the new era