Ahh, digging back, I remember now. While I did play briefly with Nick and Null (and I wanna say... Pez?) on separate occasions, the time I'm thinking about was the playthrough I did with some RL friends - who I've occasionally referenced (PPMcBiggs, Dimento, and Flaky Adam). That was when I played Gaige. That was also before all their wives cracked down on them playing video games, which is about when I guess we Halforumites started playing Left 4 Dead.
This was a game where you were trying to show @ThatNickGuy the ropes. He was playing Soldier, and I was downloading the game to join the two of you, but it took too long and you guys had signed off by the time it was done. So Cranky is the only one I've co-op'd with, or even played at all.