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The aim of this study is to enhance the design of a multilayer and planar metamaterial that serves multiple functions, including high efficiency and ultra-broadband absorption, as well as acting as a narrowband sensor. The primary feature of this absorber is its fully planar structure, which enables the flexible utilization of two distinct absorption functionalities: ultra-broadband absorption, achieved through the application of the MgF2 layer, and narrowband absorption, achieved through the implementation of the Cu layer. To conduct the simulation analyses, COMSOL Multiphysics® simulation software (version 6.0) was employed. The initial innovation lies in the fact that upon irradiation of normal incident light on MgF2 side, the material exhibited an exceptional average absorptivity of 97.0% across an ultra-broadband range spanning from 410 to approximately 2300 nm. Moreover, when the same normal incident light was radiated on the Cu side, the material demonstrated a distinct peak at a precise wavelength of 480 nm, accompanied by an absorptivity of 95.66%. Notably, these results were obtained with the added benefit of angle insensitivity. Such characteristics arise due to the multiple excitation of diverse resonant modes facilitated by the localized surface plasmon resonance and metal–insulator–metal Fabry–Perot cavity. The second innovation focuses on demonstrating that MgF2 can serve as an effective anti-reflection layer, enhancing the absorptivity of the ultra-broadband absorber. The third innovation aims to establish that Cu is the optimal metal choice. Even substituting Cu with other metals did not diminish the absorptivity of the ultra-broadband absorber; it should be noted that alternative metals might negatively impact the absorptivity of the narrowband absorber.