Can character evidence ever be an affirmative defense?

No, according to a 2002 case from the Fifth Circuit Court of Appeals:

United States v. John, 309 F.3d 298, 303 (5th Cir. 2002)

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And also from the U.S. Supreme Court:

Michelson v. United States, 335 U.S. 469, 476, 69 S. Ct. 213, 219, 93 L. Ed. 168 (1948)