Tempered endoscopy for real groups III: Inversion of transfer and $L$-packet structure

This paper examines adjoint relations for spectral analogues of the geometric transfer factors of Langlands and Shelstad in the case of the tempered spectrum of a real reductive algebraic group where the complex points are connected. Each tempered irreducible character is then expanded explicitly in terms of endoscopic characters. The analysis is also reinterpreted in terms of structure on $L$-packets in the form conjectured recently in much greater generality by Arthur. A triviality result is proved for the Whittaker normalization of spectral transfer factors which simplifies the results for certain inner forms of a quasi-split group.