I finished playing OkamiDen a while ago. It's been exciting! Okami and OkamiDen are more than just game; they are pieces of artwork!
Anyway, while playing it, I observer their distinctive outline style, and realised it's not done by the game engine, and it can be easily done in Cinema 4D.

So, I made a test scene with Cinema and Unity game engine. It can be viewed here (requires Unity Web Player. Install it!)

I wrote a blog about the technique to share. It is quite lengthy, thus I wrote it in a separate page:HERE!

Well, it's been fun doing it. I hope you enjoy it too :)
As suggested by my friend YuanQin, a Unity user, here's a direct comparison between the Polygon Outline method vs. Unity Default Toon shading. As we can see from the images below, Unity Toon Shader gives unexpected result and many artifacts in some areas, compared to the Polygon Method.
In terms of performance, Unity Toon Shader seems to be giving more stable output. In my machine, it ranges between 900-1100 fps. Meanwhile, the Polygon shading method has larger range of output between 800-1100fps. I haven't tested the method on a bigger environment with more objects.
Well, if anyone happen to use this method, do let me know the result in Unity or any game/VR engine :)