Fake Mortal allowed Fuli to modify his entire body. It was considered to be rare as metamorphic sigils usually transformed their user's body parts into that of a beast rather than an object, and because it affected his entire body. He was shown to be able to use it properly as he could change where his vital organs were located, a feat that would be disastrous if done wrong. He was also able to transform himself into other people, such as Kaji, and was able to replicate Wadatsumi's Shark Bite.
According to himself, he was initially only able to transform his fingertips, but was able to use it on his arms and legs through training. From there, he trained himself to be able to change his face, and then the insides of his body.