The Hoaxblower is witness zero. He needs a day long proctological examination under the bright lights of a Senate impeachment trial. That will expose the nakedly partisan assault on POTUS and his supporters that impeachment is.
He can't be "witness zero," because there's no formal evidence record anywhere indicating who he is.
Another witness -- probably that Sergeant Vindman misfit -- who would have to be called to testify and be compelled to identify who this "whistleblower" was. Then you move on to that jackass.
Personally, I believe that coup lawyer Mark Zaid should be the first witness called by the defense. He posted a Twitter message back in 2017 claiming that there were all kinds of insurgents in the U.S. government prepared to remove Trump from office, and he was allegedly representing the "whistleblower" as his attorney. So he should be called to testify and provide a list of all his clients to Trump's legal team.