Professor Portal
Very rare4.2%ObjectiveOptionalMulti-player
Co-operative
After completing co-op, complete Calibration Course online with a friend who hasn’t played before
You have to replay the co-op Calibration course, online, with a friend who has never played Portal 2 co-op before - that's the catch. Line someone up before they ever touch co-op; if all your mates have already played it, you're stuck waiting for a genuinely fresh partner. Play the whole Calibration course together and it pops for you. Sort this early - it's the single most awkward one to arrange late.