The CustomSteve Mod for 1.6.4 lets you play as any 3D model. This mod is based off of the Hatsune Miku character, and comes with her built in. You can also use custom animations for these models.
When using a 32 bit computer, only the Miku model will show up. On a 64 bit computer, any model can be used. This mod comes with several built in.
Custom character can be installed using PMD and VMD files. These can be found online with a quick Google search. Put the files in the .minecraft/CustomSteve\models folder.
Download CustomSteve Mod for...