Sadamitsu the Destroyer (2001)

The Earth is overrun with alien criminals known as the Ryukei-dai, and an extraterrestrial robot, the Pursuer is deployed to apprehend them. Unfortunately in the middle of a fight with an alien, the Pursuer is distracted by a human, which gets it destroyed and reduced to a mere helmet. The helmet calls out to the human, a gang leader named Sadamitsu, for help; the hotheaded but kindhearted human accepts, enjoying the thrill of a good fight but knowing when to stop. And thus begins Sadamitsu's adventures as a superhero protector of Earth. But things start to complicate upon the arrival of the Vulture, a powerful battleship deployed to destroy and not arrest the Ryukei-dai - and the Vulture holds a deep secret connected to Sadamitsu's past, that may be the key to saving the Earth.