SI MU-0 (Standard issue, military undersuit 0):

"Standardized lightweight form-fitting undersuit, issued alongside just about any currently fielded standard armor. Mainly consists of varying layers of fullerene weave, thickened around the limbs and torso, a thin semi-smart non-Newtonian fluid layer to help mitigate blunt impacts, and an active liquid cooling system woven within. More updated versions (Designated as SI MU Mod 1) link directly to the user’s neural net for increased functionality and vitals monitoring. More updated and modified versions come equipped with artificial muscle fibers. Comfortable enough to be usually worn as or under normal civilian and military clothing."

Art by Hanazawa.