The Middle Boarder
The middle boarder appears to be the leader of the three boarders who room at the Samsas' home. He's called the middle boarder because he stands in the middle when he first speaks, but oddly enough, he's still called the middle boarder when the boarders' positions have obviously changed.
Other than speaking up for the boarders, he looks exactly like the other boarders (long beard and all). It's not clear what the boarders do except mill around looking stern. As the chief boarder, the middle boarder challenges Mr. Samsa's newly re-established authority as head of the household.