Mokuba Kaiba

Mokuba Kaiba is Seto Kaiba's younger brother.

Personality
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.

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).

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 a 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 consistantly care about more than anyone or anything.

Orphanage
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.

In the second series anime, 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.

Although Mokuba was insistently happy at the orphanage, Seto had always aspired greater things for both of them and looked to the future.

Adoption
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 had won by cheating in the manga. In the anime, he 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.

Death-T


In the manga, to impress Seto and get revenge on Yugi for beating his brother in Duel Monsters, Mokuba forces him into a game of Capsule Monster Chess. As Mokuba had hoped for the other Yugi emerged for the game. Mokuba uses a rigged dispenser, to give himself high level pieces and Yugi low level ones. Dark Yugi however beats Mokuba, by tricking him into lining up his pieces diagonally, allowing "Torigun" to take them all out at once.

Mokuba later collects Yugi and Katsuya Jonouchi and brings them to the Kaiba's mansion, at Seto's request. Mokuba invites Yugi and Jonouchi to a meal. The meal being a Russian Roulette Dinner game. A rotating platform is used spun to determine who eats which meal. He poisons Jonouchi. The game is rigged as Mokuba has a hidden switch in a syrup bottle to stop the platform. Dark Yugi manages to smash the bottle, giving Mokuba a poisoned meal. Mokuba eats it and loses, handing the antidote over to Yugi. Mokuba falls over poisoned, while the butler rushes to his aid. Mokuba is somehow un-poisioned.

The next day, Mokuba faces Yugi in a game of Capsule Monster Chess, during Death-T. Seto however advised Mokuba against this, knowing he would lose. Regardless Mokuba went through and Seto bet against him, saying Yugi would die in the next stage of Death-T. After confronting Seto over this, Mokuba storms off in fury to face Yugi and prove his brother wrong.

Again Mokuba rigs the dispenser, but still loses. Seto, holding no sympathy for his brother as before, subjects Mokuba to a Penalty Game known as the "Illusion of Death", as per "Death T" rules. However Dark Yugi returns and saves Mokuba from the grisly punishment. Mokuba then explains Seto's past to him.

Thankful for Yugi saving him, Mokuba rescues Yugi's friend Honda, who had been trapped in the previous stage of Death-T.

Duelist Kingdom
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 boys deck and his starchips, 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 (cunningly disguised with a hat and a bandit-like hankerchief) 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 dissappared. Mokuba reappears next to Yugi, stealing his starchips. 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 thugs take 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. Not wanting to see Seto die, Yugi allows him to win the duel, only to see Seto lose his soul to Pegasus after Tea won Yugi the right to enter into the main castle. In the end, Mokuba gets his brother back, and Yugi wins his soul back as well.

Battle City
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. Mokuba doesn't end up persecuting 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 build a tower of boxes, and Mokuba manages to escape, but Téa is captured before she manages to. Mokuba takes a helicopter to pick his brother up.

Noah's Kingdom
Mokuba plays the role of supporting character here. He travels through the virtual world along with Kaiba and watches along with Kaiba scenes from their childhood. At the latter part of the arc, Mokuba gets kidnapped by Noah trough the help of Big 5 and gets brainwashed by him. He was last seen grieving for his step-brother.

Grand Championship
A guy trys to take control of kaiba'a company

Dawn of the Duel
Mokuba was knocked unconscious by Bakura and he wasn't seen again until the beginning of Yugi and Atem's final duel where he and Roland tracked Kaiba to Egypt.

Deck
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 Spell and Trap Cards involve kid-themed items, reflecting his naive nature.

In Yu-Gi-Oh! Worldwide Edition, he used to play a deck with only monsters and Polymerization. Sometimes he would have gained Blue Eyes Ultimate Dragon as well. He plays everything in attack mode then.