I am making a figure out of RKT and covering it in modeling chocolate. I was wondering, how hard does it get when it dries? I want to make sure that it won't stay soft and malleable when I am finished with it.

Also, how should I support it on top of a 2 tier cake? It is about the size of a baseball. I tried sticking a wooden dowel on the bottom for the beginning process so that it can dry without laying directly on itself, but the stick ended up going through the head of the figure. ( I guess I didn't compact the RKT enough and I was putting to much pressure on it when covering it with modeling chocolate.)  So how should I support it on the cake? Should I put a bubble tea straw under it and stick it to the cake with melted chocolate?

I've done several RKT objects that I covered in modeling choc.  It's dries fairly "hard" but I find it's more the heat from handling it that causes it to soften. I have modeled on dowels and I have also just placed on a cake.  In my profile, the M&M cakes are RKT covered in melted choc and fondant  that I just stuck on- no support under it...I think at least one is just on the cake, the other on the board.  In the Jersey Shore cake, the fist is covered in peanut butter modeling chocolate. It has a dowel running through the whole thing.  I hope that helps a little.

I learned a great trick recently, which is to build modeling chocolate figurines on lollipops. You can build the head around the candy and the body around the stick. I've also used styrofoam or paper balls inside the heads, which although not edible, make the heads nice and light-weight so they don't rip through the stick. The stick extends out of the base of the body because it is eventually used to hold the figurine securely on the cake.

For stability while I'm working on figurines, I jam the sticks down into a block of styrofoam (sealed in plastic wrap), and that holds everything upright. It helps to build the bodies and heads separately then add the heads to the bodies only once the features are complete. Below are the beginnings of some bodies made for sitting down with legs draped over the edge of the cake. 

Modeling chocolate will get a little dry crust on it over time, as long as the environment is dry. If it's humid out, it tends to remain sticky. Lots of humidity can cause it to grow sticky and will make thin pieces go limp over time. If you want your figure to set up as hard as possible, leave it out for a day or two in a cool dry room. Then keep it sealed in an airtight container or on a sheet pan covered in plastic wrap until it's ready for use.