The [Muscular Expansion - Brutal Amplification], according to the system, would unlock extreme muscular potential, transforming the user into a gigantic and muscular version, something Reiji associated with the Hulk's transformation.
A/N: For those that may say he is reckless by not hiding enough from the Devils but that is intentional. He is making them curious, he is not showing them enough to know about him he is showing them enough to wonder about him. He already knows their characters and knows they will eventually find out about it, so he is steering that eventual meeting in the direction he wants instead of trying to avoid it.