Linchpin
"A linchpin is an unassuming piece of hardware, something you can buy for sixty-nine cents at the local hardware store. It's not glamorous, but it's essential. It holds the wheel onto the wagon, the thinger onto the widget.
Every successful organization has at least one linchpin; some have dozens or even thousands. The linchpin is an essential element, the person who holds part of the operation together. Without the linchpin, the thing falls apart.". ~ Seth Godin