What is a word for someone who is not the sole decision maker?

For example, a parent can be the decision maker over a child’s actions if under the age of 16. They can be a sole decision maker, active in making decisions for the child.

But, what is someone who does not make all the decisions – kind of like a co-parent, but more general. Co-decision-maker. More passive, someone who consults another individual before making the decision or works in harmony with someone rather than taking control and making all the decisions.