In my terminology (which is a mishmash of classic literary criticism, hero's journey theory, dramatica and TV Tropes) this kind of character is a "Dark Horse Hero". I'm not sure which of the above sources supplied the term.

The Dark Horse can be either a jerk on the same team (Han Solo) or even the villain (Darth Vader) at first, but they become heroic at the end (although they don't always stop being jerks.) Note that the more evil they are at the beginning of the story, the more likely it is that they die in the act of redeeming themselves (Darth Vader again).
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Regards,
Eric Fretheim

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

"Success is the ability to go from failure to failure without losing your enthusiasm." ~Winston Churchill

"I write one page of masterpiece to ninety-one pages of shit. I try to put the shit in the wastebasket." ~Ernest Hemingway