A ringer
A ringer is someone who plays with your clan in a big match but isn't even part of the clan. They're just there to help you win.
"That guy is a ringer. He's not even in our clan, but he showed up and helped us win."
"She's a ringer. She's not in the clan, but she just showed up and took out half of the enemy team."
"He's the ringer. He's not even in the clan, but he's here to help us win and then disappear."