Linchpin

[Sarah]

A pin used to prevent the wheel of a vehicle from sliding offthe axletree.

[Sarah]

Literally; n., a [pin] or [keeper], placed through or around an axle, that prevents a wheel from slipping off of its axle.

Slang; n., Any part or person that is so vital to something that if it [went missing], the whole thing would fall apart.

[Sarah]

linchpin - the faggot of the group who thinks he is important.

[Dave Paprocki]

The person in a group of friends who keeps them together, is forgotten about, but once leaves, [things fall apart].
The person in a group who is irreplaceable and once is lost, things go to shit.
The thing that can link multiple groups together, but once is removed; things fall apart [like a house of cards].
That one person in the group who was quiet, was picked on by all but put up with it. The [underappreciated] friend and person who was always there. But once was removed... the great Friends-War starts.

[Vania]

A person or thing vital to an enterprise or organization.

[Sonia]

the most important member of a group or part of a system, that holds together the other members or parts or makes it possible for them to operate as intended: