supporting but indispensable role; major supporting role
Originally a kabuki term for a character who moves the plot forward while appearing secondary; now used broadly for a person or element that is not the main focus but is essential to the story or outcome.
He's not the lead, but as a major supporting role he drives the story forward.
この事件の狂言回しは、意外な人物だった。
The unexpected person turned out to be the one pulling the strings behind this incident.