User blog comment:Jakson212/What is Zer0?/@comment-5015744-20130917193023

We have no idea, there was once one rumor that the devs said he/she was a robot but there was no source for that, in reality no-one knows who Zer0 is, as no dev has confirmed any detail about his/her true identity, I would presume they will reveal it in Borderlands 3.

Also Zer0 did reveal his true self to Tannis, presumably due to that she was crazy and would not have her mind blown OR would be so disreputable as to not be believed (or perhaps other reasons).