Tagged under Ecchi.
Characters not wearing anything at all. They may use bed sheets or other clothes to cover up - however, those don't
count as proper clothes since they aren't part of the character's outfit.
Tag is generally considered part of the Ecchi category, except for images with artistic nudity (where nipples or genitalia aren't drawn explicitly; if they are shown, they are presented in a non-sexualized manner).
See also: Bottomless, Partially Dressed, Shirtless, Undressing, Wardrobe Malfunction.