What is the most critical factor in the agency-client relationship?

The most critical factor in terms of having client and agency relationship is the chemistry of both parties such as the chemistry of both the client and the agency in terms of personal things. The reason why it is the most critical is that if both have a disruptive chemistry, it does not make a strong bond to each other, making the relationship critical and hard to even purse of working together.

answer; (i-o) psychologist

sounds good to me mate!