It is worth noting that this is a huge spoiler for the series. If you do not want to know the result for one of the characters, be sure to turn away now. That said, if it helps, I will only answer this question with a yes or no response. No, Zenitsu does not become a Hashira in the series. As for why that is, you will have to either read the manga right now or wait for the anime adaptation to finish to see why.