Download the SinED toolset
, place makepak in the base directory, open a command prompt (or dos prompt), go to the base directory and type:
makepak -u pak0.sin
"Apparently Ritual made a nude skin for Elexis (the female character in Sin) and hid the skin in /models/pl_elexis/folder-x . This should be where the .def files are. Rename pl_blade to a temporary name. Change pl_elexis to pl_blade and change your temporary folder name to pl_elexis. Run Sin and go to the multiplayer character select menu. BEWARE: Blade will wear the Elexis skin, so be careful when you change to the Elexis character, or you could delete pl_elexis. Change the character select menu to Elexis and change the skin to blade_base5.tga. And Elexis is nude."
Get more skins here