key man
key person/individual/executive, linchpin, number one, numero uno, leading character, leader, chief, chief cook and bottle washer, pivotal person, cornerstone, focal point, axis, heart, prop, center, main ingredient, vital part. Consider using "key" alone as an adjective or noun: she's absolutely key to the project, he's the key to this transaction. Or, choose an appropriate adjective: crucial, critical, pivotal, essential, vital, indispensable. The insurance industry uses "key man" when insuring the top executive. If you need to use the term, put it in quotation marks.
