There are many advisors and here is an attempt to disambiguate them. You have a major professor, PI, or a guiding professor that is often called your Advisor, but lets not call them that anymore, because it confuses things. There is also the Head Graduate Advisor, currently Mary Firestone, that is the Departmental Graduate Advisor. Most forms need to be signed by the Head Graduate Advisor. You need an Advisor Code to register for courses each semester and you get that from your Graduate Advisor, which is not the same as your PI. The Graduate Advisor is a faculty member that is not on your dissertation or orals committee, but they are responsible for approving your course selections, and the breadth requirement.

You also want to have a Guiding Committee formed (you will do the forming) as early as possible. They will eventually approve your coursework and are likely to evolve into your orals committee.