I had one for my wife a few yrs ago. What ever class I was in she was in. It did not matter how I got there. For example, If I used the last award business class seat for myself, she would still get a business seat even if they only had full fare Business left.
Or, if I received an upgrade she would upgrade as well. This all could have changed by now.
I think you can stack them as well. ie, add the kids. If you never travel alone with your kids, you might want to make the companion tide to your spouse, so (she/he) can take the kids to the in-laws without you
I think you can also change the companion name quarterly.