You could say that Doc Brown and Marty McFly's main enemy is the clock because if they don't tie up all their time-traveling loose ends before the deadline, everything goes horribly awry (see "Back to the Future Part II"). But the one human always making their job harder than it needs to be is Biff Tannen –- a towering, dimwitted oaf who gets through life by cheating and intimidating others into doing what he wants.
In the first movie, Biff terrorizes the high school version of Marty's dad, George, until George finally gains the courage to slug him for forcing himself onto Lorraine (Marty's future mom). When Marty returns to the present, he finds a more confident version of his dad, being waited on by doting manservant Biff.
But justice is fleeting. In the second movie, Marty and Doc have to jump into action after their antics result in present-day Biff getting ahold of a sports almanac from the future, which he uses to amass an enormous sports-betting fortune and take over the town like he's Mr. Potter from "It's A Wonderful Life." In "Back to the Future Part III" (granted, a '90s movie), Biff's great-grandfather, Buford "Mad Dog" Tannen, kills Doc Brown, forcing Marty to head back to the 1800s to set things right.
Biff is the kind of character you love to hate but also love to watch. And you can thank Thomas F. Wilson's dependably hilarious performance for that.
