SA1_Chars aka Dreamcast Characters Pack is a mod for SADX that replaces the character models in the DX version of the game with the models and textures from the Dreamcast version of the game. It is only compatible with the ModLoader, and requires either the 2004 PC version or BetterSADX to play.
Open the contents of the 7z from the release page and drop the contained folder into the mods folder in your SADX install. Select it in the ModLoader and load AFTER any other Dreamcast mods you may be using.