Wigner multiplets in QFT: from Wigner degeneracy to Elko fields

We establish the theoretical foundation of the Wigner superposition field, a quantum field framework for spin-1/2 fermions that exhibit a Wigner doublet -- a discrete quantum number arising from nontrivial representations of the extended Poincar\'{e} group. In contrast to the previously developed doublet formalism, which treats the Wigner degeneracy as a superficial label, the superposition formalism encodes it directly into the structure of a unified field via a coherent superposition of degenerate spinor fields. By imposing the Lorentz covariance, causality, and canonical quantization, we derive nontrivial constraints on the field configuration, which uniquely identify the Elko field as the consistent realization of the Wigner superposition field. Our analysis further clarifies that although the Elko field is a spinor field, it possesses mass dimension one and obeys the Klein-Gordon rather than the Dirac kinematics. Moreover, we explore the general Elko representation through basis redefinitions, showing that certain traditional properties, such as being eigenspinors of charge conjugation, are artifacts of specific basis choices rather than intrinsic features. Finally, we discuss the physical implications of Elko as a dark matter (DM) candidate. This work lays the foundation for a systematic reformulation of Elko interactions and its phenomenology as a viable component of DM.