Masaru Aso is the main protagonist of the movie Kamen Rider ZO. Transforming into a grasshopper mutant known as Kamen Rider ZO, he seeks to protect a young boy from monstrous creations that have been sent after him.

Masaru is capable of transforming into Neonoid #1, a mutant that partially incorporates the grasshopper gene and machine called ZO, who would be later called Kamen Rider ZO. Masaru first gained this ability after being forced to undergo surgery and genetic modification by Doctor Mochizuki, who was driven by madness. At first, he had no control over the transformation and it was triggered by emotional stress. A bolt of lightning somehow corrected this problem while putting Masaru in a coma. During his four year coma, the Red Core on his waist absorbed the biological energy of nature to enhance ZO's power to an unknown limit.

Having undergone organic remodeling, Masaru has a strong body and special abilities even before transformation. He has extraordinary durability, as seen when he withstood a direct hit from Doras's Moleculaser, and was able to transform immediately afterward.