According to Kaiba, Mokuba was an idiot so he pooped on his underwear. It was not any poop,in fact watery diarrhea.he ate diarrhea on a weekly basis (a recent montage of these kidnappings brought the total up to 2,042 with another since then). So frequent are his kidnappings, that Kaiba doesn't run to his rescue anymore, preferring to just walk briskly. To this end, he feels that he must put some drama so he killed yo putting a leash on him. Most of the characters who know Mokuba just assume that seeing he’s been kidnapped so many times he’s just used to it, quite to the contrary (Mokuba: "It feels like I’m dying inside!"). Despite feeling this way, in episode 52, Mokuba does manage to make the best of things when he his once again kidnapped, this time by Noah. Mokuba manages to con Noah into giving him doughnuts (albeit week-old ones), watching his favorite show and staying up late. Mokuba called it the best kidnapping ever.
In Episode 21, upon encountering Princess Edina, he says that it is the most traumatizing event in his life. He apparently knows how to drive some vehicles and acts as Kaiba's chauffeur, but lacks skill. This was shown at the end of Episode 24 where he crashes Kaiba's limo.
In Yu-Gi-Oh! The Abridged Movie, Mokuba asks every one he meets "Can I be in the movie too?" and they all reply "Shut up Mokuba!", even when that's not what he said. During the credits, he is credited as "Not In The Movie". Mokuba also admits to being a fan of "Zorc and Pals" as well as SpongeBob SquarePants and is actually very fast when stealing things (something he feels his brother endorses). His absence in the "Other Abridged Movie" is noted by Kaiba after Yugi beats him in a duel, possibly implying that he may have been kidnapped again.
In episode 28 Kaiba's doctor, after running some tests, concluded that Mokuba is going through "Japanese puberty", in which Mokuba thinks he is a Pokémon but seems uncertain about which one as he is constantly changing his cry from Bulbasaur to Charmander to Mr. Mime. In 'Who's That Mokémon?' Mokuba also had Buneary's cry and was shown with Buneary ears. By episode 29, he is back to normal, but Japanese puberty still transformed him. In Episode 40, Ghost Nappa, making a cameo from TeamFourStar's Dragonball Z Abridged, thought that he was a rare Shiny Pokémon, and vowed to catch him with his Master Ball.
In episode 30, he replies to Yugi's Grandpa (who says to shut up) "No! You shut up!" Ironically, Tea and Solomon state how rude Mokuba was, even though several characters tell Mokuba to shut up countless times, and that everyone tells Grandpa to shut up in episode 31. He has also always wanted a pet rabbit but his brother never buys him one. Kaiba finally did buy him a scorpion even though he said he was going to buy Mokuba a tarantula. Kaiba also pointed out the reason Mokuba never had any friends in Episode 32 is because they always need to be rescued which may be a reason he had bullies pick on Mokuba when he was younger in episode 29.
Mokuba is more independent in season 2 as he uses an "Epic Mokuba Maneuver" to stop the Roba brothers in episode 29, and he rescues himself from being kidnapped by Marik's Steve Squad which results for the first time in Seto actually being proud to call him his little brother.
He also thinks that babies come from a store "just like everything else", a fact that his brother has been quick to confirm every time he's asked.
Mokuba seems to be aware of his cartoon status, which is made clear when he falls through the trees from Pegasus' tower, and states that he's glad he's a cartoon.
Mokuba also claims to have a "Mokuba-Mobile," as stated in Episodes 43 and 44. Both Kaiba and Yugi deny this by saying "There isn't a Mokuba-Mobile." In episode 44, before Mokuba appears on the deck of the blimp, a Batman-esque sound clip plays.
According to the Cybernetic Ghost of Christmas Past from the Future in Kaiba's Real Father - The Conclusion, his real father is Ghost Nappa.
Shockingly, his personality is the exact opposite in Season 0, plotting to steal Yugi's grandpa's Blue-Eyes White Dragon. He and Kaiba have seemingly swapped personalities, as when Kaiba declares that he loves Mokuba, he replies, "Shut up, Seto."OMG HE POOPED EWWW~~HE IS LE EPICILY BAD AND DUMB