Delete hidden admin account?

What hidden admin account are you referring to?

By default there are no hidden user accounts on the Mac.

User accounts may be added or removed by going to System Settings... > Users & Groups.

You must have at least one user account with admin privileges to properly run and manage the computer.

I think you should take a look at the suggestions in this article, as they show the way to identify the biggest Boat-Anchor sized files. One known, you can decide what to do about them.

What is “Other” storage on a Mac, and how… - Apple Community

the terminology has evolved, but the principles are the same.