Hardening behavior with its ultimate deterioration (or softening) after uniform deformation limit and strain rate sensitivity of metals at elevated temperatures are characterized. To avoid non-uniqueness in the identification procedure based on only simple tension test, an inverse identification procedure involving not only the simple tension test, but also the creep test is newly proposed. Moreover, for better understanding of creep behavior, analytical and numerical creep models are developed for the generalized multiplicative constitutive law accounting for hardening, softening, and strain rate sensitivity. Robust parametric studies are presented to explore the effect of material and process parameters on creep deformation behavior, which is also effectively used for the two-step characterization procedure of the proposed hardening model. The identification procedure for the multiplicative constitutive model is demonstrated using AZ61 Mg alloy sheet, which provides good correlations between calculated and measured stress-strain curves and creep strains under different strain rates and prior-creep strains.