Mokuba Kaiba is Seto Kaiba's younger (biological) brother, and one of the few people devoted to Seto Kaiba. Their biological mother died giving birth to Mokuba, and their father died only three short years later. When Mokuba was about three years old and Seto nine, Seto used his wits and will to beat Gozaburo Kaiba in a chess game, with the stakes being that Gozaburo would have to adopt him and Mokuba.
Seto became Kaiba Corporation's next heir, while Mokuba was often left to his own devices. Nonetheless, it was Mokuba believing in his brother no matter what that allowed Seto to turn the tides against his wicked adoptive father and take over Kaiba Corporation with 51% of the stock.
Manga Mokuba vs. Anime Mokuba
In the manga, Mokuba has a vastly different personality from how he appears in the NAS-produced series known as Yu-Gi-Oh Duel Monsters. While he shares his brother's smarts and love of games, he's not as cold and emotionless as Seto Kaiba is known to be, nor as ruthless. He cares for his brother so much that he would even "betray" him if it meant saving him in the long run.
In the Yu-Gi-Oh Duel Monsters animated series, Mokuba is spoiled and rude, but he generally gets along well with Yuugi and Anzu. He still makes fun of Katsuya Jounouchi and Hiroto Honda, though. He is also considerably more kind-hearted than his brother and his anime-counterpart; in the anime-only Virtual World saga, he felt sorry for his would-be foster brother, Noa Kaiba, even though the other boy exploited him and tried to take over his body. Earlier in Battle City, he was also the first to try and intervene to try and save Anzu Mazaki's life when Malik Ishtar and his Ghouls threatened in during a duel between Possessed Katsuya Jounouchi and Dark Yuugi. Anzu had previously saved Mokuba from the Ghouls, and it is thought that he likes her best out of all of Yuugi's friends.
In the original Toei animation, Mokuba's personality more closely follows the manga; he is extremely rude, but just as devoted to his brother. He even helps Seto bring Yuugi and his friends to Death-T, Kaiba's specially-created amusement park meant to spell the end of Yuugi's life and dueling days.
Birthday: July 7th, 1985 (Cancer)
Grade: Primary/Elementary School, Unknown Year (Most likely 6th year/6th grade, if he is 11-12 years old, or 5-6 years younger than Seto Kaiba)
Height: 142 cm (approximately 4'11")
Weight: 28 kg (approximately 61.6 lbs.)
Hair Color: Black (with green highlights in Toei animation, dark-blue highlights in Yu-Gi-Oh Duel Monsters series)
Eye Color: Blue-Violet (Duel Monsters)/Light Green (Toei)
Blood Type: O
Favorite Food: Chocolate Parfait
Least Favorite Food: Celery
Voice Actors: Katsue Miwa (Toei Yu-Gi-Oh! series), Junko Takeuchi (Yu-Gi-Oh Duel Monsters series), Tara Sands/Tara Jayne (English dub seasons 1-4), Caroline Lawson (English dub, season 5)
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