Talk:Hashiras/@comment-39992552-20190923010326/@comment-43912082-20190923151839

They are not always nine, at the moment, there is not a thunder pillar, but Kuwajima was one (Zenitsu's master). It's impossible to have two pillar of the same breath tho, so the pillar would take the second one to make him his disciple, or what they call Tsuguko (their successor). Also, like the page says, to become a pillar, you need to take an extreme training and kill one of the 12 moon demons or kill 50 demons.

Hope that answered your question ! :)