Hiroshi Umino is a young boy who leaves his birth world Earth to make a life for himself away from the planet's overbearing conditions. During his journey meets the infamous lady pirate Emeraldas. The Pirate Queen, admiring the boy's wishes presents him with the Cosmo Dragoon once owned by Tochiro. This begins Hiroshi's journey that changes depending on the media.
In the manga, Hiroshi possesses the standard appearance of characters designed by Leiji Matsumoto, a mostly ovaloid body featuring a disproportionate head. The OVA however gives him a more evenly balanced face, giving him a look more similar to most major male characters. Hiroshi has wavy brown hair that in the OVA sometimes covers part of his face. He is always dressed in a jacket worn over a shirt, pants, and shoes. The OVA also gives him gloves.
Hiroshi is determined to make a life of his own, having no one else to support him after a long time. As such he is willing to take any job if it can get him what he wants and put up with most of the cruelty that surrounds him. However, there are limits to how much he can take as he yelled down at his boss' spoiled daughter for her haughty comparison to Emeraldas. No matter what tragic cruelties, Hiroshi always presses on to achieve his ambitions all the while enjoying life as he can. His strong will and ambitions amidst the cruel world has earned Hiroshi the respect of people in places he has visited and even Emeraldas who helps Hiroshi from a distance and even gives him a Cosmo Dragoon in recognition of his passion.
Hiroshi while young has demonstrated that is more than capable than what is suggested of him. He built his own ship and is able to pilot the same ship to reach areas of space that most other ships are not. After receiving a Cosmo Dragoon, with enough experience and practice Hiroshi is able to use the weapon at the best of times, hitting his target at the right place and time.
Hiroshi at an unspecified time had left Earth for reasons that have not been outright stated, but seeks freedom. In the manga, he builds a homemade spaceship to escape. He crashes onto Mars's moon Deimos near a canyon. Admitted to a hospital, Hiroshi gets out to reclaim his ship while encountering Emeraldas who came to the crash site. After exchanging words, Emeraldas gives Hiroshi advice for him to get his wish.
Hiroshi then begins working to get enough parts to get his ship working while dealing with a few issues including his Ganymede boss's spoiled daughter who after arguing with her loses his initial job. Getting a new job in the mining operations, he makes friends with one of his coworkers after an accident involving an attempt to cook cup noodles. His boss also helps him out since his daughter is his own responsibility. However his boss's daughter tries to kill Hiroshi by sealing him in the mine shaft. Hiroshi is saved in time by his boss and later has his own daughter executed by the authorities. With the same authorities after Hiroshi for owning an unregistered ship, his boss gives him needed supplies while slowing them down at the cost of his life with Hiroshi departing Ganymede for the stars.
Hiroshi eventually encounters Emeraldas again who lets Hiroshi onto her ship. When Hiroshi asks what Emeraldas is searching for, she replies that she is looking for a certain man before handing Hiroshi a Cosmo Dragoon. Emeraldas eventually reveals Hiroshi's real name: Zero Banno, the son of an explorer who was executed for starting an uprising in a forced labor camp. Hiroshi being from a disgraced family left Earth in search of freedom and a new chance at life. Emeraldas had also mused that she and Hiroshi were destined to fight and that a secret between them would be the trigger. Unfortunately the manga ended with no clear meaning to this.
In the OVA, Hiroshi is a stowaway who sneaks aboard a freighter in order to leave Earth. As the freighter is flying through space they are attacked by a ship, there he makes friends with an elderly man. However, the train is saved by a mysterious and incredibly powerful spaceship and is able to make it to their destination, Heavy Melder's Gun Frontier. Seeing Emeraldas in action, Hiroshi is greeted by the Pirate Queen who asks what he hopes to find. Hiroshi states his wish for freedom. Emeraldas, sympathizing with Hiroshi gives him the Cosmo Dragoon owned by Tochiro Oyama. He eventually gets mixed up with Emeraldas and her battle against Baraluda and the Metanoids.
Hiroshi's surname means 'Sea Field' while his given name means 'Wide/Open'. This refers to Hiroshi's wish for freedom. His true surname in the manga, Banno, when read in ban'ya means 'Barbarian', referring to his family status in disgrace while his name Zero refers to wanting a new life from nothing.