sparring partner
1. Literally, someone with whom one practices martial arts or boxing. OK, John here is going to be your new sparring partner. He's tougher than you're used to, but he'll help your skills improve.
2. By extension, someone with whom one engages in a debate, argument, or heated discussion, especially when one enjoys doing so. I was sad to hear he wouldn't be in the class next semester—he was my favorite sparring partner during class discussions. The two have been sparring partners on the morning talk show for nearly ten years.