Marlon de Quevedo, known in the original Japanese version as Teppei Takamiya, is the main protagonist of the anime B't X. He grew up on Kamui Island, a private island north of Hokkaido, where he worked as a farm caretaker. After the death of their parents, he was raised by his older brother Kotaro Takamiya, a brilliant robotics engineer. When Kotaro left for Germany to study, Marlon stayed behind. During this time, he encountered a mysterious wounded woman named Karen, who became his mentor and trained him in combat for roughly five years in preparation for a future crisis. Karen also gave him a powerful high-tech gauntlet called the Messiah Fist, which was derived from a cell of the ultimate B't known as Raffaello.

The story begins when Marlon reunites with his brother at a robotics convention in Mechatopia, China. The convention is attacked by agents of the Machine Empire, and Kotaro is kidnapped. Marlon pursues his brother's captors into the Empire's hidden fortress, the Area, located in the Gobi Desert. During a fierce battle with the lieutenant Metal Face, Marlon is gravely wounded and thrown into a scrap heap. His blood spills onto the damaged body of a dormant B't named X, a quilin-shaped artificial intelligence machine originally belonging to Karen. The blood revives X, and the two form an unbreakable bond, with Marlon becoming X's second blood donor. Together they escape the Area, beginning a long journey to rescue Kotaro and confront the Empire.

Marlon is characterized as kind-hearted, brave, and noble. He is driven by a deep love for his brother and a strong sense of justice. His initial motivation is simply to save Kotaro, but as he travels deeper into the Empire and learns about the threat posed by Raffaello and the Machine Emperor, his mission expands to protecting all of humanity. He embodies themes of protection, resilience, and the transformative power of bonds between humans and artificial beings.

In terms of abilities, Marlon relies on his physical strength and martial training, but his true power is unlocked when he rides B't X. While mounted, his striking force increases dramatically, allowing him to deliver attacks at supersonic speed. His primary weapon is the Messiah Fist, which he can use on its own or in combination with X. Karen also provides him with a special disk that activates a transformation called Battle Gear. When he inserts the disk into a slot on X's neck and commands Battle Gear On, both he and X are enveloped in a combat armor: Marlon gains a suit of armor, and X gains enhanced capabilities, including larger wings and new techniques. Over time, Marlon develops a deeper synchronization with X, unlocking additional powers such as the Break On attack, which possesses the energy of the sun. An important plot point is that Marlon carries a fragment of the sun within him, which ultimately proves vital in countering Raffaello's destructive evolution.

Marlon's key relationships include his brother Kotaro, whose rescue is the central driving force; Karen, his mentor and the original donor of X; and X itself, which starts as a reluctant partner but becomes a loyal companion. As the story progresses, Marlon also earns the alliance of several former enemies, including the Four Guardian Spirits of the Machine Empire, who defect after witnessing the threat of Raffaello. His growth is marked by his increasing ability to cooperate with X and his maturation from a simple farm boy into a determined hero willing to risk everything for the greater good.