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Good question, unfortunately no one but KT knows but obviously the tradition didn't start with the first.
I think Unohana resigned from her position after she fought Zaraki. "Kenpachi" is the strongest warrior of an era, after Unohana saw that there was someone more powerful than her, she resigned knowing that Kenpachi would come and seize the title one day.
Probably there was a time when there was no kenpachi, and then there was someone else who was battle-crazy enough to become the 2nd. In that sense, we could call Unohana the 0th Kenpachi and number the rest from 1.
Incidentally the 2nd Kenpachi should be the weakest of the lot, since every one after him gained the title by killing the previous holder, so they should have been getting progressively stronger through history. Even Zaraki killed the 10th when he was at a level where he was a joke compared to Unohana. Now after this 'training' session, he's presumably the first and only one to have ever surpassed her.
---------- Post added at 01:03 AM ---------- Previous post was at 01:01 AM ----------
I wonder where does the kenpachis come from ? Do they get promoted or do they come from the streets and kill the current kenpachi and claim the title ? and why do they claim the title of kenpachi when clearly there are stronger captains besides them and why isnt yamamoto a kenpachi ? maybe a criteria to be a kenpachi is to have a pure melee zanpaktou like ikkakus and their zanpaktou has to have the current strongest melee type zanpaktou to be kenpachi ?
From the last chapter it seems to me that Unohana doesn't even acknowledge any of the other nine. I suppose from her POV since the 2nd one never killed her, he barely counts as a Kenpachi. Only Zaraki actually counts in her eyes, if I understood her sentiment correctly.