Joseph Yaht is a twenty-three-year-old man who serves as an assistant to the prospector Sid Munzer in the Americas on Earth. His origins lie with the Maba clan, an ethnic minority from the region of Adeska. His family was killed by other people from Adeska, and he was abandoned in the mountains as a child before being found and raised by Sid. This background as a member of a persecuted minority has a lasting impact on his worldview. He possesses red hair, green eyes, and his fashion sense, including a pink blouse and a choker, is considered unusual.

Joseph is characterized by a stubborn, brusque, and somewhat exclusive personality. He is a proud and fierce individual who does not take well to being contradicted. His difficult past instills in him a strong sense of independence and a deep-seated feeling of being discriminated against. This leads him to act with the belief that if he cannot rise above others, he will never improve his own station, driving him to be quite belligerent and confrontational. He frequently attacks or speaks ill of those he sees as above him, including the Moonrace like the protagonist Loran Cehack, as well as Earth nobles. This rivalry with Loran is a notable part of his demeanor, as he holds a sense of competition against him. Despite this abrasive exterior, he is described as having a good heart and does not put others down simply for being different, with his attitude being rooted in a personal struggle for self-respect rather than pure malice.

Joseph's role in the story evolves significantly. He begins as an excavator of mobile suits, using his mechanical knowledge gained from working with Sid. He later becomes a pilot for the Inglessa Militia, commanding the Jalopy Squad, named after the recovered FLAT mobile suit he pilots. His motivations are largely tied to his desire to rise above his past and prove himself, especially as a representative of Earth in the conflict with the Moonrace. This desire culminates in a major act when the Turn A Gundam is stolen; Joseph uses his initiative to successfully recover the powerful mobile suit. Rather than return it, he declares that the fight is now for the Earthlings and pilots the Turn A Gundam himself, though the cockpit is not properly configured for him, limiting its performance.

Key relationships shape his personal development. His surrogate father figure is Sid Munzer, who took him in and trained him. His most significant relationship is with Fran Doll, a Moonrace woman who teaches him how to pilot the FLAT. Despite his initial prejudice against the Moonrace, he falls in love with her, and this relationship gradually softens his attitude, though his belligerent nature remains until the very end. The two become a couple, and by the latter part of the series, Fran is pregnant with his child. Before the final battle, he entrusts Fran to Loran and dramatically announces the pregnancy, a risky act that also demonstrates his continuing reckless side.

Joseph possesses a notable set of abilities. He is highly intelligent and has extensive knowledge of machinery from his time as an excavator and assistant, allowing him to serve as a maintenance manager and instructor. He also has sharp observational and deductive skills, at one point suspecting Loran of hiding something and realizing Fran's true identity as a Moonrace member. As a pilot, he first operates a FLAT and later commands his squad, culminating in his temporary piloting of the Turn A Gundam.

After the war, Joseph retires from the militia. His ambition to rise up in the world is apparently not entirely extinguished, but his life takes a new direction as he lives with Fran and raises their child, effectively becoming a househusband while Fran works as a journalist.