RMD

RMD is a variation of RenderWare's node-based model format. It comprises all of the 3D assets in the PS2 versions of Persona 3 and Persona 4. Using Amicitia v1.6, it's possible to edit textures, rearrange animation order, and import/export rigged meshes. RMDs contain all of the model's textures and animations in addition to the mesh.

Amicitia
Featured article: Amicitia (program) Amicitia is TGE's all-in-one editor for Persona formats. Currently, it supports almost everything that RMDEditor can and more. It also features texture conversion from PNG (for both TMX and RMD). You can get it here. In order to import a rigged custom model in Amicitia, yours must be skinned to the bones of the original model you're replacing. Export it as a triangulated ASCII 2011 FBX. Right click the Clump0 node and choose Replace. Also be sure to right-click the Textures node and Add the textures the new model uses.

RMD Maxscript
Using 3dsMax, you can run TGE's RMD Maxscript to load skinned meshes, textures, bones, and even animations from an RMD or a file containing an RMD (such as a PAC). You can get it here.

Trivia

 * Other games including Sonic Heroes, Shadow the Hedgehog and Grand Theft Auto III customized versions of the same format as well. A list of games that use Criterion Software's RenderWare model format can be found here.
 * Grand Theft Auto III uses the same Animation format, referred to as "hanim animations."