There are plenty of tutorials and rigs for Blender all over the intertubes - many of them make rigging look like hard work. Some of them even tell you it is. But actually rigging a character for use with (pre-existing) mecanim animations isn't that onerous at all.
The trick is to make sure you're in the correct mode in Blender when creating and editing the bones and meshes. We threw together a quick rig in Blender, exported the model into Unity and had it picking up the mecanim animations in next to no time!
I guess we all have to start somewhere, so this is how I did it. It might not be "right". It might differet to others do it. It might even be the same. But this is how I got it to work.
First things first, we need a mesh. This is a character I nicked from an existing Asset store (though modified and with some UVs mapped to it). With the mesh in position (at 0,0,0 and rotated to zero) we need to add some bones. Let's start with the first one.
In object mode, menu Add - Amature - Single Bone
And, like magic, a bone appears.
If the bone appears hidden, or inside the mesh, we need to make it visible from all angles, by ticking the "x-ray" box in the object tree. With the bone selected, go to the tree panel and select the object tab, Scroll down the list and tick the x-ray box. The bone should now be visible, no matter which angle you view your model from.
With the single bone selected, change from Object mode to Edit mode using the little menu at the bottom. Select the head of the bone and place it roughly where the neck will start. Place the tail of the bone below where the hips will end.
Now here's where one of the really nice things about Blender comes in. Instead of trying to learn and remember a million and one keyboard shortcuts (some of those online tutorials just give a stream of Ctrl+ this and Alt+Shift+that) you can simply hit the space bar.
A pop up menu lets you type in what you want. The search results are (usually) context sensitive. So with just a few keystrokes, you can usually find what you're looking for.
With the entire bone selected (but still in edit mode) hit space and type "sub"
We're going to split the back bone into three pieces. Mecanim likes bodies split into three. So we choose armature - subdivide (notice how the search results change if you have a different object selected, or are in a different mode)
And change the number of cuts to two (we want the backbone to be in three pieces, so need to cut it twice)
Now we need to create some legs. Before going any further, let's turn on "x-axis mirroring" so that as we build the armature on one side of the character, Blender does the same amount of work for us, on the other side.
In edit mode, with any bone selected, go to the "options" tab in the tools panel and select X-axis mirror. Now, any operations we perform on our rig will be repeated on the opposite side of the character.
Let's start with a leg. Select just the node between the hips (lowest bone) and the abdomen (middle bone). If either of the entire bones light up, you need to select just the connection point, not the entire bone. Hit the space bar and type "extrude". Select Armature - Extrude. You can see that there's also a keyboard shortcut to achieve the same - just press E to start an extrusion.
Move the mouse around and when the new bone is in the right place, simply left-click to create the new (extruded) bone. Now select the new bone entirely, and from the tree panel, select the "bone" tab. Scroll down and untick the "connected" tickbox.
Now we can move the new leg bone around without affecting the rest of the bones. A dotted line shows us where the bone originated (and its parent) but by disconnecting the bone, it doesn't have to start/end connected to a previous bone.
Now- what about that x-axis mirror? Where's that gone?
Well, for that to work, it requires our bones to be named using a specific convention. Basically if you move a bone that ends in .L or _L, then any bone with the same name but a .R or _R suffix are moved/scaled/rotated at the same time. So you can name the first bone, with the suffix _L then create a second bone (extruding from the abdomen as before) and rename it with the suffix _R.
Then, as you move/rotate one bone, you should see the other bone reacting. An alternative approach would be to use Shift+E to extrude the bone in the first place. Doing this actually creates two bones - one with a _L suffix and one with a _R suffix; so as you move/scale one, the other updates in realtime.
With two leg bones - each with the suffix _L or _R - we need to subdivide these, just as we did with the backbone. This time, we split each leg into just two parts, with a single cut.
Then with the root of the leg selected, Shift+E to extrude a new bone towards the toe (or use the space menu and type Extrude on each leg, renaming each new bone with the appropriate _L and _R suffixes). It may be easier to switch to ortho-left view (numberpad 3). Select the new bone and subdivide into two.
Next, with the tip of the top backbone selected, Shift+Extrude the arm bone(s).
And subdivide into four parts (three cuts). The first part of the bone-chain is the collarbone. The second is the top of the arm or bicep. The third bone is the lower arm or forearm. The fourth bone is the hand controller.
Our character has fists, not fully articulated hands. So we're not going to bother with hands, fingers and thumb bones - a single bone to rotate the entire fist will suffice for our animations. Move the connection points between the bones to the correct places for the shoulder, elbow and wrist joints.
Don't forget to rotate the charcter and make sure that the bones are fully inside the mesh - not just from the front view, but from the side and top views as well. Depending on the mesh, you might benefit from disconnecting the shoulder joint/upper arm bone from the collarbone(s).
Lastly, turn off x-axis mirror and extrude the top of the backbone into the head. Sub-divide to create two parts. Move the connection between the two new bones to create a neck bone and a larger head bone.
Don't forget to shift into side-view to move the bone end-points forwards and backwards, especially when positioning the neck and head bones.
While it may be prudent to re-name bones as we go, we tend to do all the bone renaming at the end, working systematically through the entire skeleton, renaming each bone in turn.
Our bone names are:
- head, neck
Mirrored bones are renamed, but using the .R suffix.
At this point, you can go into Pose mode and move the skeleton around.
But the mesh isn't rigged to the skeleton. To do this, we go into object mode and select our mesh first. Then shift-click to select the bones (the entire skeleton should highlight, indicating you're in object mode - if just a single bone lights up, you're probably still in edit or pose mode!)
On the keyboard, hit Ctrl+P and select Set Parent To - Armature Deform - With automatic weights. Now, as you move the bones around, the mesh should follow.
Save the file.
Copy it to your Unity project Assets folder.
After starting up, Unity will automagically import your new Blender character. Select your Blender character from the project panel and in the Inspector window, click the "Rig" button. Set the type to humanoid and hit "configure".
After a few prompts to save and apply changes, you should see how well Unity has imported the new model. If all has gone well, the entire character should be green.
Hit the Muscles & Settings button to check that the character deformations are correctly applied when the bones are moved around.
If your character is correctly rigged (and it should be if you've followed the steps carefully) moving the sliders should animate the floating character properly. When done, scroll down the inspector panel and click the "done" button.
Now, drag an instance of your new character into the Unity world stage. Assign a mecanim controller and apply any mecanim-compatible animation. Hit play and watch your new character come to life!
Here's our character playing a full cycle of fighting animations, created by a third-party.