Kenshi Masaki is a fifteen-year-old boy from Earth and the main protagonist of the story. He is the younger half-brother of Tenchi Masaki, born to father Nobuyuki Masaki and his second wife, Rea. His heritage is unique, as his father is a distant descendant of Juraian royalty, while his mother is an artificial human originally from the world of Geminar. Kenshi was sent from his home on Earth to the parallel universe of Geminar as part of a mission programmed into his mother to combat a great evil known as Gaia. Before his departure, he was hypnotized by the genius scientist Washu Hakubi, a family friend, to prevent him from revealing the secret of his alien family to strangers.

In terms of personality, Kenshi is defined by his hardworking nature, kindness, and quiet determination. He is a remarkably capable individual who approaches every task, whether it be combat, household chores, or manual labor, as a challenge to be met with enthusiasm. He is respectful and refers to the many women in his older brother's household as his sisters. He can be trusting and somewhat naive about the effect his actions and demeanor have on others, as his strength and sincerity often lead to the women he befriends becoming infatuated with him. Despite the attention, he remains focused and somewhat oblivious to overt romantic advances, showing no clear signs of romantic interest himself throughout the story, though hints of a mutual crush with a girl named Yukine Mare are present. A notable quirk is that when startled, Kenshi exhibits certain vocal and physical characteristics reminiscent of the cabbit Ryo-Ohki, a companion from his childhood.

Kenshi's primary motivation is not a grand ambition, but rather a desire to return home. Initially tricked into participating in an assassination plot, he is captured after refusing to kill his target, the young ruler Lashara Earth. He then accepts a position as her servant, which provides him with a purpose and a path forward on this unfamiliar world. As he navigates his new life, his motivations gradually expand to include protecting the friends he makes and understanding the conflict brewing on Geminar. His role in the story is that of an unwitting but pivotal figure in a prophecy, recognized as a legendary Seikishi, or Sacred Mechanoid pilot, due to his unique white mecha. His extraordinary piloting skills, martial arts prowess, and swordsmanship make him a decisive force in the escalating war.

Kenshi's relationships are central to the narrative. His primary connection on Geminar is with his master, Princess Lashara Earth, who comes to depend on him heavily. He also forms close bonds with her bodyguard Chiaia Furan, who becomes like an older sister to him, and her own older sister, Mexiah Furan, also known as Doll, who develops deep feelings for him. He shares a mutual respect and a budding romantic connection with Aura Shurifon, a noble knight, as well as with Yukine Mare. He is a kind and loyal figure to all, treating everyone from princesses to servants with the same genuine consideration. His actions and character eventually lead him to become the captain of the airship Swan, which he declares an independent nation.

Throughout the story, Kenshi demonstrates significant development. He begins as a confused and lonely stranger in a new world, having spent months in isolation and desperation before the series begins. Over time, he builds a new life for himself, earning the trust and affection of those around him and finding a place where his unique abilities are valued. He matures from a boy following orders to a leader who makes critical decisions, ultimately confronting the great evil Gaia directly. A key part of his character arc involves his hidden potential, as his white Seikijin can, in moments of rage, transform into a powerful but destructive black berserker form, fulfilling a prophecy that the warrior of light could also be a warrior of darkness.

Kenshi possesses a wide array of notable abilities that make him a formidable force. His physical capabilities are superhuman, granting him immense strength, speed, and endurance, allowing him to perform feats such as catching thrown knives in mid-flight. He is an incredibly skilled swordsman, so much so that the expert warrior Mexiah admits he is better than she is, and he is a master of martial arts. He is also surprisingly adept at domestic skills like cooking, cleaning, and construction, as well as wilderness survival. As a Seikishi, he is a peerless pilot, capable of single-handedly defeating entire squadrons of enemy mecha. His own Seikijin is distinct for its white color. He later obtains a powerful sword named the Tenchi-ken, the same name as his brother's blade. At the climax of his journey, he is able to channel the sword's immense energy to generate a Light Hawk Wing, a powerful divine energy construct, to defeat Gaia, though he cannot generate this power on his own.