The macroscopic mechanical behavior of brain white matter, including softening and damage, is well characterized experimentally, yet its underlying microstructural mechanisms remain unclear [1]. Most existing representative volume element (RVE) models consider only axonal fibers in a matrix, limiting their ability to capture contributions of glial cells and interconnections to tissue mechanics [2]. In this study, we investigate how microstructural organization—including axonal fibers, matrix, and glial cells—affects macroscopic stress–strain response. A three-dimensional RVE is developed, explicitly resolving the hyperelastic matrix, beam-like axon fibers, and a deformable glial cell. The RVE is subjected to periodic boundary conditions and macroscopic loading to homogenize the effective stiffness tensor. The model is first calibrated with fibers only to reproduce literature-reported white matter stiffness [3]. The glial cell is introduced as a separate hyperelastic phase, tied to the surrounding matrix, to quantify its effect on apparent stiffness and local strain amplification. Interconnections between microstructural components are subsequently modified to allow progressive debonding under loading, enabling exploration of microstructural damage mechanisms. The study provides: (i) a validated parameter set reproducing effective white matter stiffness; (ii) characterization of glial cell contributions to composite response; and (iii) mechanistic insight linking axon–glial interactions to macroscopic softening. It opens a physically motivated route to incorporate cellular-level damage into white matter constitutive models, enabling predictive multiscale simulations relevant for brain injury assessment.