How to get hip/buttocks armature to bend correctly When my model bends forward and squats down, its buttocks disappears. I'm wondering how I can get the buttocks to look more like the red line in the attached picture.

I've tried weight painting, but at best I can get the buttocks to be flat, not curved like the red line.

Any help is highly appreciated. Thank you.
You need extra bones to controll the inner part of the gluteus, the lower part of the butt and the buns. All of those extra bones are child of the leg. Then you can further deform the butt according to the rotation of the leg. Most principal you need to have distributed weights between all 4 new buttock bones.