Tā suīrán bùshì zhǔjué, què yīzhí wèi péngyǒu yáoqínàhǎn.
He wasn't the main player, but he kept cheering his friends on from the side.
Tips
history
From the Yuan zaju 《两世姻缘》 by Qiao Mengfu — 你这般摇旗呐喊,簸土扬沙 ("you're there waving flags and shouting, kicking up dust"). Originally the foot-soldier's role in battle: not fighting, but making noise to back up others.
usage
Often slightly deflating when applied to adults — suggests the person isn't the main actor, just a loud supporter. Neutral to positive when applied to fans/cheerleaders.