A ringer
A ringer is a fake player who shows up to a game just to make everyone else look like amateurs.
"That bowler is a ringer. He's got the skill of a pro, and he's playing in our league like it's a joke."
"She brought a ringer to the match. We were all amateurs, and she just waltzed in and won."
"He's the ringer. He's been hiding in the back of the room the whole time, and now he's going to blow us all away."
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