Kiyomori, made by the always amusingly named TMSuk Corp, is a 37 DOF robot clad in traditional samurai armor, as his namesake the Heian-era warlord Taira no Kiyomori often was.
They're making a lot of noise about how "Kiyomori's knees are flexible". I think this is either marketing misunderstanding, or else he uses some form of compliant/series elastic actuators on the knees. I'm always in favor of using series elastic actuators in robots - it simply mimics the human muscle/tendon combo much better. But I don't think that feature is the breakthrough here.
The bigger thing I think is that he has more than one degree of freedom at the hips. A long-strided walk around the room will probably convince you fairly quickly that your hips need to allow your legs to both swing (thigh moves forward-back) and also pivot (toe points left-right) in order to walk smoothly. In addition, I also believe that your tailbone needs to allow your pelvis to rotate relative to your torso. And if you want to get really anal about the analysis, your ankle probably needs to allow your shin to lean inwards and outwards slightly as well. Most robot designs allowed the leg to swing, but that's pretty much it. Hence, very stiff walking.
It looks as if TMSuk called in the expertise of Waseda U's WABIAN-2 group to help them out with this project. WABIAN-2 is a bipedal humanoid robot that Kiyomori seems to be derived from, and has a surprisingly competent waist/hip design that allows him to walk and move much more like a person. A characteristic evident even in still photographs:
Here's a video of WABIAN-2 in action.
It's interesting to note that structurally speaking, WABIAN-2's leg "bones" are basically two long plates. This is a pretty good mechanical design in a lot of ways. It's light, has good rigidity, allows plenty of internal space to mount actuators, and makes joint design dead simple. From a hobbiest perspective, it is also extremely easy to manufacture. Anyone who can download the CAD software from EMachineShop can have aluminium plates of just about any dimensions made to spec. A more conventional "dog bone" mechanical design is a lot harder to make (large bones milled down from even larger blocks of metal = $$$$), weighs a lot more, has no internal space, and the joint design is almost always harder. Better robot designs have used the parallel plate model with excellent success. It falls down when you need to have more than one degree of freedom at a joint, but otherwise it's an excellent way to do things.