Anti

A sentient Kaiju with the capacity to copy all of Gridman's powers, he assumes the form of a human boy when not in combat. He was created to counter Gridman and would even interfere with Akane's monsters for that sole purpose. In his human form, his weapon is a handheld buzzsaw.

[Spoiler] Despite Anti's insistence of being a Kaiju, Gridman refuses to kill him as the youth slowly develops into a sentient being rather than a creation empowered by Akane's hatred. After being disowned by Akane, Anti becomes committed to find a new purpose and his action of defending Gridman results in the former's evolution as Gridknight. [/Spoiler]

(Source: Wikipedia)