In all my terminal sessions on OS X or macOS, if I type who, in addition to the expected users (all me in various windows) I also always see
_mbsetupuser console ...
Just who is that, and what is he/she doing?
As the name suggests, it is a process associated with setup, which, in this case is running as a result of the upgrade from 10.11.x to 10.[11|12].x+y or from 10.12.x to 10.12.x+y (and may also appear in upgrading from older versions to 10.11.x). This process does not appear after an update to 10.13.x.
Unfortunately, though the "About This Mac" dialog may say that your version is 10.[11|12].x+y, you are in fact effectively between versions, and will get all kinds of odd behavior (repeated requests to unlock the Keychain, connectivity problems, wifi issues, mail synchronization failures, process crashes, etc.) until you complete the installation process, which you should be able to accomplish with a reboot.