clavimere
noun
Someone or something without which an action is pointless or impossible.
“Hey, Gaston called. He won’t be able to make the movie."
“Okay, we’re down one, then. Wait, Gaston? He’s the one who wanted to see this movie in the first place - and he’s the only one who’ll be able to explain the stinger after the credits. He’s the clavimere; we can’t go without him. Let’s reschedule.”