MakubeX was found in a sack as a child. Only his last name, 間久部 (Makube), is known, for it was written on the bag itself. He later came to be known as MakubeX, the 'x' symbolizing an unknown variable, and his "name" is always written in romaji. MakubeX is a genius hacker, capable of manipulating computers to do whatever he pleases. He was originally taught by Gen but swiftly surpassed him in skill.
He was raised by Gen, whom he calls Grandfather, but left home to join the VOLTS. During the VOLTS's rule of Infinity Fortress, he was one of the "Four Kings", alongside Shido Fuyuki, Fuuchouin Kazuki, and Masaki Kurusu. After Ginji Amano left the fortress, he, as the only one of the Four Kings remaining, took over, instituting a dictatorial rule; he was known at this period as the "young demon king".
During the 'Infinity Fortress' arc, the GetBackers are hired to retrieve the mysterious 'IL' from him. It is later discovered that IL is in fact a computer disc containing important data required for producing an atomic bomb. (however, IL is also a short form for "implosion lens", one of the most crucial parts of an implosion-type nuclear weapon). The arc ends with his reconciliation with Ginji, who acknowledges the transfer of power.
MakubeX's second-in-command is Sakura Kakei. In the IL arc, his four chief subordinates were Juubei Kakei, Haruki Emishi, Takuma Fudo, and Kyoji Kagami; Fudou vanished afterwards, but Juubei and Emishi remained with him. In the manga, after the Divine Design arc, Toshiki Uryuu came under him; he assigned the newcomer to his old comrade, Juubei.
In the Manga
At a young age, MakubeX had discovered that all actions in Infinity Fortress were in fact controlled by the 'god' of Babylon City, the top area of the fortress. Realizing this, MakubeX decided to give the god an edict: Turn back time to the time of the rule of Volts, or Infinity Fortress will be destroyed.
In the manga, it is noted that he, along with Ginji, are the "Children of Babylon City's God". He has also recently discovered his own latent special powers in the Lost Time arc. During his losing battle with Sakura, MakubeX was able to manipulate Infinity Fortress's virtual space at will using an "Omega Theorem" without the use of his trusty computer. MakubeX had by then realized how he was a virtual program, a part of Infinity Fortress itself. Knowing this, MakubeX attempted to sacrifice himself to the Archives and erase its memory bank and AI from within - much like a computer virus. However, he was stopped by Ban, saying that MakubeX's loss would serve no purpose but to further enrage Ginji/Raitei into destroying Infinity Fortress and everyone in it as well.
He is especially close to Sakura, as shown in the Lost Time arc when he was forced to fight her while she was under the influence of the Black Thread; many fans have speculated that their relationship is romantic, despite a significant age difference. He is also fond of Haruki Emishi, and treats him like family. The "Hide and Seek" championship in Infinity Fortress at the end of the Kiryuudo arc was organised by the boy genius for Emishi.
At the end of the Lost Time arc, he gains the ability to leave Infinity Fortress, thus implying that he has become an average teenager, although he seems to have retained all his computing skills. He was last seen bumping into the majority of the cast while shopping with Sakura.
In the Anime
Due to the differences of the second Infinity Fortress arcs between the manga and the anime version, MakubeX's role also changes. MakubeX's kidnapping prompts the GetBackers' return to the Limitless Fortress in the final story arc of the anime.