Mystery solved. Well, sort of. At least I am not alone in my affection for this unassuming guerrilla art. The little guy is called Stikman (yes, without the "c"). And here is a Washington Post article about him.
I have found Stikman in Porter Square, Harvard Square, the Haymarket, the North End, Government Center, and near the Park Plaza Hotel.
I noticed the white one several weeks ago when I was crossing a street near Porter Square. I only recently discovered the black one on the other side of the square. They are painted right in the middle of the crosswalk.
I notice these things. And sometimes I get reminded that the world is still a marvelous place.