Oakland A's - Bart Simpson - Reliable bad-boy winners who march to the beat of their own drum. Locked in a constant struggle against the overbearing establishment. Every time you think they're going to get what's coming to them, they weasel out of it and surprise you again.
Florida Marlins - Snake - Riding high for a while, publicly executed, and then somehow cheated death to escape and rise again. Important players in cities known for their rampant drug and crime problems. They seem to be in trouble a lot, but never really go away. Almost lost something of great value to a much wealthier, more prestigious individual, but hung onto it in the end (stealing his girlfriend back from Mr. Burns, 2003 World Series).
Houston Astros - Marge Simpson - Their fortunes rise and fall with those of a guy who has put together an unbelievable string of successes despite obviously being an inveterate jerk. Seemingly infinite reservoir of patience for someone who has proven themselves to be not worth hanging on to (Homer, Brad Ausmus).
Cincinnati Reds - Principal Seymour Skinner - Spent much of their existence under the hand of a domineering, insane woman who was impossible to please (Agnes Skinner, Marge Schott). Possessors of a dirty little secret that they would rather sweep under the rug (Skinner's true identity of Armand Tamzarian, Pete Rose). Their lives were given meaning in the '70s (Vietnam, the Big Red Machine) but now all they have to escape the monotony of their everyday existence is the flashbacks.
San Francisco Giants - Abe "Grandpa" Simpson - Even older than you would have thought. Surrounded by other decrepit crones, and they have a tendency to live in the past. Completely dependent on a big lug of questionable moral turpitude who, curiously, never seems to get any older.
Los Angeles Dodgers - Disco Stu - Overconfident and forever predicting the return of their glory days. Brought up by devoted fans more often than is probably reasonable, considering their sporadic appearances. Haven't been relevant since the '80s.Hat tip to Baseball Think Factory.