In the US, there are very few exceptions where you can stay anonymous in the court system. Basically, unless your life is in immediate danger, you must use your legal name and people that initiate the case also has to use your legal name to prove subject matter jurisdiction.

It’s by design. It’s a popular tactic DMCA abusers use. They file DMCA under corporate entities names, and then you have to use legal name to dispute it, and then they have your legal name to harass you.