A non-constitutional rule of evidence specifying when a person is or is not a competent witness cannot override a federal constitutional right to have a potential witness testify.

Cage v. Lousiana, 498 U.S. 39, 41 (1990) ("It is plain to us that the words ?substantial? and ?grave,? as they are commonly understood, suggest a higher degree of doubt than is required for acquittal under the reasonable doubt standard. When those statements are then considered with the reference to...A jury instruction that the jury may acquit only if it has a grave and substantial doubt about guilt is unconstitutional.

United States v. Hinton, 423 F.3d 355, 360 (3d Cir. 2005) (holding that out-of-court statements of a 911 caller which identified defendant as the assailant was erroneously admitted because "statement....The right of confrontation guaranteed by the Sixth Amendment to the Constitution of the United States bars the introduction against a criminal defendant of testimonial hearsay by a nontestifying declarant unless (i) the absent declarant is unavailable to testify at trial and (ii) the defendant had an adequate opportunity to cross-examine the absent declarant prior to trial.

United States v. Fox, 473 F.2d 131, 134 (D.C. Cir. 1972) ("The defendant himself, however, may, if he chooses, offer evidence of his good character by presenting witnesses attesting to his reputation in the community").A criminal defendant may offer a pertinent trait of his or her character circumstantially -- to show the defendant's doing or non-doing of a specific act.