For me, a good teammate means that he will discuss constantly, mainly with arrows on the board. He will signify what moves he'll make, and get feedback from his teammate. This is what I always do, and the results are fantastic when both teammates do this.
Almost as important is courtesy. Not getting upset at blunders, handling a loss with grace, not swearing, and even being kind after a win rather than gloating. Whenever I play, I make sure to say "gg" at least, and I also apologize when I realize that I made a bad move. I try to be as kind as possible no matter the outcome, and hope for the same in my teammate.
This thread is for discussing what you value in your teammate, as well as what qualities define you which would help a teammate. This thread is meant to work in tandem with the Find a Teammate thread, so that you can connect with players in the rating range and time zone you're interested in, but also can find players with a personality, play style, or sportsmanship that will make your team succeed!