A. Phex Brothers

Personality
Abilities

Rock Physiology
While similar to a human's, Yotsuyu's body can change into rock. Because of this, he does not need to sleep or rest. He can mimic rocks to the point that he is indistinguishable from them.

Norisuke theorizes that Yotsuyu's physiology requires him to have at least a small amount of skin respiration and in the absence of such his body structure will begin to break down due to it becoming less structurally stable than an ordinary human's.

Also it is curious to mention that Yotsuyu was able to produce a strange transparent film from his eyes/mouth which he used to waterboard Yasuho but at this stage it isn't confirmed whether this is a trait all 'Rock Humans' possess or something exclusive to Yagiyama.

Trivia

 * Yotsuyu's unique rock physiology and Norisuke's inferences at how he could be another species altogether mirrors the characteristics of the Pillar Men from Part II: Battle Tendency. Josuke had even intended to trap him permanently underwater like Magenta Magenta before him, in the same way Kars was blasted into space.
 * He shares the the same occupation as an architect with Masazo Kinoto.
 * Like Aishou Dainenjiyama, Yotsuyu's surname is a reference to sets of mountains in the Sendai region, which is the real world location of Morioh and the home city of Hirohiko Araki.