This is a big question that will probably bring many different answers. Some would consider unconditional love being the foremost important quality a spiritual worker should have. While I don't disagree, I found through the many years of being a spiritual worker, there are three qualities a spiritual worker must posses before they can have a successful path to serving humankind. These are dedication, commitment and personal responsibility.
Dedication, as a spiritual worker, you must dedicate yourself completely to the spiritual work. There is no wiggle room for half measures effort. It is either you do it completely devoted to it or not do it at all.
Commitment, as a spiritual worker, one is committed fully to your spiritual duties come what may. Life is a mosaic of pleasure and pain. While it is important to be committed to developing pleasure with your spiritual work, it is even more so important to be committed during the painful process a spiritual worker endure. You do not get to pick when to be spiritual or not.
Personal Responsibility, I believe this is the most important quality. You must accept personal responsibility for your action. You must behave responsibly and with integrity, to be that trustworthy individual who will keep all your promises and always show up for the spirit world ready to do spirit work.
Can one be a spiritual worker without these qualities? I imagine so, but without these qualities, your mission will not be as beneficial as it ought to be.