undersuit
アンダースーツ インナースーツ インナー ピッチリアンダー ギアインナー
When a character is not wearing multiple outer garments that who normally wears, and exposing the inner clothing that is not normally exposed (or partially exposed), but it is not underwear only.
Or the undershirt, bodysuit under clothes, etc. are almost completely exposed with no outer clothes worn.
If the character is simply not wearing one of the outer garments (like jacket, cape, etc), please mainly use no_xx (no jacket, no cape, etc), unless the outer garment usually covers a large area of the inner clothing.