Comment by NekkoDroid

The system owner. Usually that is the same entity that owns the secure boot keys, which can be the person that bought a device or another person if the buyer decides to delegate that responsibility (whether knowingly or unknowingly).

In my case I am talking about myself. I prefer to actually know what is running on my systems and ensure that they are as I expect them to be and not that they may have been modified unbeknownst to me.