Mr.clean

I kind of have the opposite opinion of this song as everyone else. I think Mr. Clean is the one who has started using drugs.

I think the line "Mr. Clean, tell me, is that really you?" signifies disbelief of the singer who knew him prior as someone who was very clean.

Also, certain drugs often have a tendency to make users especially angry, violent, and irrational, which would account for the lines "I wonder what you've done to become so big and dumb," and "last time we met you beat me up."

And finally, when he says "who... shows the world he's incomplete," I think it means that Mr. Clean does not feel whole, so he uses drugs as a crutch to make him feel whole.

That's just my take on it. Especially because Millencolin never seemed like a hard-core, druggy band that would criticize being sober.