At the beginning of the manga, Mokuba is very mean and will go as far as trying to trick Yugi Muto to get back at him for defeating his older brother, Seto Kaiba. Mokuba is another good game player who specializes in Capsule Monster Chess ("Capmon"), but is not as good of a gamer as his brother. He is so committed to avenging his brother's humiliation, he uses several extreme measures, such as having his friends brandish a machine gun, a knife, and a stun gun in one manga chapter to coerce Yugi to play Capsule Monster Chess with him.
In the second series of the anime, Mokuba is eternally devoted to his brother Seto Kaiba and is constantly by his side. He is more naive and good-natured, and will do whatever his brother tells him to. He also seems to trust Yugi and his friends more than he did previously. There is a part in one episode when he saw (Yami) Yugi watching him from another building when he was about to kick some brothers out of the Battle City grounds, when he considered, "You know, I bet Yugi would give them another chance!" He must really respect Yugi as he starts to think like him.
In the original Japanese version of the Yu-Gi-Oh! manga, Mokuba tends to omit honorifics when referring to people other than his older brother. He also has a habit of calling Jonouchi "Jōnouchi no yarō"; (that bastard Jonouchi). Mokuba politely refers to his older brother as "nii-sama" (honorable older brother). He is very forgiving, especially to his brother. Even after Noah hypnotized him, and brainwashed him into forgetting who Seto and the others were, Mokuba forgave him and treated him like a brother (they are actually step brothers)
In the second series Yu-Gi-Oh! anime (Yu-Gi-Oh! Duel Monsters), Mokuba is a kind-hearted boy who is naïve to the ways of the world. He succeeds in befriending Yugi and his friends; his brother does not approve of this. Mokuba does not duel in the first series anime. He only duels in the second series and manga. Mokuba has an unlucky streak of being kidnapped by Seto's enemies forcing Seto to show his kinder side and rescue him, and is, in fact, the only person his brother is shown to consistently care about more than anyone or anything.
Main article: Mokuba Kaiba (manga)
At a young age, Mokuba and Seto were orphaned. Their actual surnames are unknown. Their mother died when Mokuba was born. Their father died supposedly in an accident when Seto was eight. Their relatives used up their inheritance and left them at the city orphanage.
Seto was bright and intelligent, which led to many prospective parents wanting to adopt him, but Seto swore to protect his brother at all costs, and wanted Mokuba adopted with him, which all had refused. Whenever bullies would pick on Mokuba, Seto would always stick up for him, including jumping a child to get back a toy plane that he had taken from Mokuba.
Although Mokuba was insistently happy at the orphanage, Seto had always aspired greater things for both of them and looked to the future.
When the former head of Kaiba Corporation, Gozaburo Kaiba, arrived at the orphanage to perform a publicity stunt, Seto saw the chance to challenge him to a game of chess, with Gozaburo forced to adopt Seto and Mokuba should he win. Mokuba was five at the time. Seto won by studying Gozaburo's chess methods.
Six years later, Seto took over the company. In the second series anime, it showed that he had used Mokuba to do so. Seto planned gaining 51% of the company in order to overthrow Gozaburo. Seto deliberately leaked this information to Gozaburo and accused Mokuba of telling Gozaburo. Mokuba sided with Gozaburo, who allowed Seto to gain 49% of the company, believing that Mokuba would give him his 2%. Gozaburo was fond of making his enemies think they were winning against him so they would look worse when he beat them, which Seto based his plan around. As Seto planned, Mokuba switched sides last minute giving his 2% to Seto.
Mokuba was originally imprisoned by Pegasus on a high tower in Duelist Kingdom, in order to force Seto Kaiba to come and duel Pegasus to rescue Mokuba. However, Mokuba escapes from the tower by a rope of knotted bedsheets, which only go a small way down the tower. After a rather nasty fall, Mokuba steals a boy's deck and his star chips, leaving him to be kicked off the island. Yugi and his friends find the boy and promise to help him. They go back to the scene of the crime. Mokuba jumps out of the bushes and challenges Yugi to a Duel. The only cards he played were Man-Eating Plant and Krokodilus. Mokuba's identity then gets found out, and he explains that after Yugi beat Seto in a Duel, his brother had gone away and abandoned him, and that it was all Yugi's fault.
Yugi begins to preach, but he looks up and realizes Mokuba has disappeared. Mokuba reappears next to Yugi and steals his star chips. Yugi gets Mokuba to see the error of his ways, and they all run to try and not let the boy get kicked off the island. All they succeed in doing is getting Mokuba captured again. Yugi agrees to duel for his freedom, and wins. But Pegasus's thug, Kemo, takes Mokuba anyway, and lock him in a deep, dark dungeon.
Seto reaches him, and just as he is about to free Mokuba, Pegasus shows up and steals Mokuba's soul, forcing Seto to duel Yugi in a largely unfair match with Mokuba's soul on the line. Seto gains an advantage through Crush Card and Blue-Eyes Ultimate Dragon and is set to win, when Yami counters with an indestructible wall of Kuribohs through Multiply, keeping his Life Points safe. Yami then backs him into a corner by "fusing" his Mammoth Graveyard with Ultimate Dragon, making it decay and lose 1200 attack points each turn because they couldn't properly fuse. In desperation Seto backs away to stand at the castle's edge, telling Yugi he will die with his dragons, implying that if he couldn't save Mokuba, his life wasn't worth living. (In the English version, Seto backed on to the ledge, knowing Yugi wouldn't attack him and let him get hurt, therefore letting Kaiba win the match). Not wanting to see Seto die, Yugi allows him to win the duel, only to see Seto lose his soul to Pegasus after Téa won Yugi the right to enter into the main castle. In the end, Mokuba gets his brother back, and Seto wins his soul back as well.
Seto and Mokuba come back from Duelist Kingdom and go back to KaibaCorp. Seto goes straight to the Big Five to get revenge on them for what they did to him and Mokuba. They manage to convince him that they did what they did to help Kaiba, not hurt him. To prove this, they tell him that they have finished the virtual world stimulator that they had been working on. Kaiba, who became very eager to test it out, decided to wait to fire them after he tested the stimulator. Mokuba was very against it, but Kaiba decided to do it anyway. While inside, it became very apparant to Kaiba that the Big Five had rewired the stimulator and he became caught by a ninja.
Mokuba, realizing it was a trap, swiped Kaiba's deck from the chair and ran to get help. He recruited Yugi and his friends to help him. They went to Kaiba's personal lab for more chair stimulators to get into the virtual world. Mokuba, Yugi, and Joey entered the world while Tea and Tristen stayed behind to stand guard.
When inside, they first enter a graveyard where they run into 3 monsters. Mokuba shows Yugi and Joey how to summon a monster. They then run into a fairy Named Iru that leads that to an old town. In there, Yugi suggests they split up to look around the town for clues about Kaiba. An old lady told Mokuba that she saw someone with a prisoner going towards a mountain and the only way to get there was to cross a large desert.
They try crossing the desert that’s after the town but get hit by a twister. They all end up back at the town with Mokuba shaken from the twister. An old man tells them that to cross the desert they need a special monster, a Niwatori card. In order to get it they must enter a tournament and win against the champion, Madame Butterfly. Joey goes up against the champ using the name Senor Porky. The champion turns out to be Mai, and then she realizes it’s Joey she decides to help them out by getting the card by force and the running for it. While crossing the desert they run into a monster but Joey gets rid of it.
When they finally reach the temple after the desert Mokuba runs towards it, desperate to see his brother again. when everyone catches up to him they open a door inside the temple and see a maze. they soon hear a girl scream, making Joey run to help her. However, they are all ambushed by labyrinth tanks and only manage to escape thanks to Yugi using Magical Hats. they then see Iru again and it leads them to a princess named Adina, who looks exactly like Mokuba (except a girl). They then find the exit but it is blocked by a Gate Gaurdian. Yugi and Joey defeat it using polymerization on Red-Eyes Black Dragon and the Summoned Skull and then adding Dragon Nails.
Adina, thankful that they had saved her, took took them to her castle where they discussed the Heroes of Legend. Soon, however, the castle is surrounded by monsters. One of them captures Mokuba, who was dressed up like Princess Adina and then taken to the castlein the sky, to where Kaiba is.
Upon meeting his brother, Mokuba set him free and gave back his deck. Kaiba disapproved of Mokuba asking Yugi for help and wasn’t thankful to Yugi, Joey, or Mai. Unfortunately, the Big Five decide to make the Mythic Dragon appear even though there were no sacrifices. The dragons first target is the person who doesn’t have any monsters to defend himself with: Mokuba. Joey, however, ordered his Red-Eyes Black Dragon to take the hit therefore losing the game and disappearing. Then Mai gets hit and is taken. Yugi tries to convince Kaiba work with him by summoning his Blue-Eyes Ultimate Dragon but Kaiba refuses. The Mythic Dragon goes to attack Kaiba, but Mokuba, the only one realizing that the attack was for Seto, pushed Seto out of the way but got hit, leaving his brother devastated.
In the end, Kaiba and Yugi worked together and defeated the mythic dragon. Afterwards, Adina turns out to be the Mystical Elf, and brought back Joey, Mai, and Mokuba. Reunited at last , everyone leaves the virtual world.
During the Battle City tournament, Mokuba is the Battle City commissioner, calling violations and penalties when needed. While his brother duels, Mokuba wanders around town. The first violation he sees is performed by the Roba brothers. While the eldest, Espa Duels, his brothers use binoculars to see his opponent's hand. Mokuba tackles Espa's brother, and is about to throw Espa out when the younger Robas tell Mokuba about how their big brother protects them from bullies by winning duels. This reminds Mokuba of his relationship with Seto, and he does not disqualify the Robas, commenting that big brother stories get him every time.
As Mokuba walks away, he's set upon by rare hunters and kidnapped. He is tied up and dangled from a helicopter, which flies over his brother's head. The Rare Hunters make Seto duel with Yugi against infamous tag team duelists Lumis and Umbra. The loser of the Duel would plummet to their death (In the English anime they would be sent to the Shadow Realm) While they duel, Mokuba is taken to a warehouse, and locked up there with Téa. Together, they construct a tower of boxes, and Mokuba manages to escape, but Téa is captured before she manages to escape. Mokuba takes a helicopter to pick his brother up.
During the Virtual World arc Mokuba and his brother are taunted by Noah as he shows them images from their past. Later on, Nesbitt, disguised as Tristan, kidnaps Mokuba where he is held captive by Noah while Kaiba faces Lector. After Kaiba wins the duel against Leichter, Noah brainwashes Mokuba and he forgets who Seto and his friends are, in an attempt to split them up. In the duel between Kaiba and Noah, Seto is about to win the duel when Noah uses Mokuba as a shield by putting him between Noah and Noah's monster, so that Mokuba would be harmed by the attack should Seto choose to do so. Kaiba is unable to bring himself to harm his last living family member, allowing Noah to get the lead in the duel. Eventually Kaiba manges to break through to Mokuba using his "Blue-Eyes White Dragon" card and when Mokuba runs away from Noah Kaiba activates Last Turn, Kaiba picking Blue-Eyes White Dragon and Noah his Deck Master which turns into Shinato, causing Seto to lose the duel. Mokuba runs to his brother after he lost but unfortunately, Noah turns them both to stone before they can be reunited. After Noah's defeat against Yugi, Noah tries to help the gang to escape from the Virtual World after Gozaburo reveals his plans. However, Noah tricked all of them so he could escape using Mokuba's body. However, after a change of heart, Noah comes back to the Virtual World to stop his father from taking Seto's body. Then Mokuba, Seto and Yugi leave the Virtual World before it explodes.
Again, Mokuba continues to be a supporting character. He's also present when Kaiba and Joey have their duel and witnesses Joey putting up a good fight, but ultimately Seto wins. He also questions his brother's emotions as well and convinces him to be there when Yugi faces Marik in the finals.
Mokuba is a supporting character through this arc. He is with Kaiba during his duels with Alister and is present during Kaiba and Yami's duel with Dartz.
Mokuba later wears a red sleeveless shirt and black pants and blue gloves with his fingers throught the holes.
Mokuba lost his soul to the shadow realm by Bakura by some unseen means and he wasn't seen again until the beginning of Yugi and Atem's final duel where he and Roland tracked Kaiba to Egypt.
Mokuba was not seen using his own Deck in the anime and manga series, but in the video games, he would play with an animal themed Deck. Some of his Magic and Trap Cards involve kid-themed items, reflecting his naive nature.mokub