In real life, if there was a masked vigilante whose codename is ‘Watcher,’ he’s a non-lethal masked vigilante who is a witness to a murder. The detectives and prosecutor want to put the vigilante on the witness stand, but the vigilante’s secret identity is, well, secret, and he doesn’t want to reveal it. Can he still testify with his mask on or testify anonymously?
It’s important to note that once in court to testify, they’re going to investigate anyone testifying.
Both the prosecution and the defense. Depending on whose witness they are, one would be investigating to make sure you’re credible and the other to hopefully find something to discredit that.
Further while vigilantism is not necessarily illegal on its own, it’s generally (almost always,) going to involve other crimes.
Vigilantes do not have privileged arrest powers, so at best one is going to have to either work a case (legally,) and then turn things over to cops, or do crime (best the shit/kill the bad guy and pray they got it right.)(they probably haven’t.)
This means that by the time a vigilante takes the stand; they’ve done crimes and even if the entire system lets you … you’re gonna get arrested.
The only way out of that is to get outed and get immunity.
All of that said, if you were a bystander and not necessarily involved, you might be able to testify as your identity and not alter ego. You would have to explain what you were doing there, though.
And you’ll still be investigated… and probably found out.