Katsura Hajime is Rei's younger brother who is said to have been kidnapped by Leviathan.
Hajime's mother died when he was very young, and he lived with his older brother Katsura Rei. Hajime cares for his brother a great deal, and the two are close. Due to his intelligence, Hajime was kidnapped by Leviathan and made to write code for him. When Hajime found out his evil purposes, he fought back and sent his creation away, hiding him from Leviathan.
Despite not being written in kanji, Hajime can mean "one" (一) if viewed as a nanori reading. Together with Rei, which can mean "zero" (零), they are a reference to binary code.
Hajime was kidnapped and transformed into data by Leviathan, who wanted him to use his intelligence to create an app that could start up any program. Hajime did this, and the Appmon connected to the app was Bootmon. When Hajime realized that Bootmon was part of Leviathan's plan to change all humans into data, he programed an emergency escape program into Bootmon and sent him away, whith instructions to hide. Leviathan changed him into a Sleepmon as a punishment, and he slept in that state for several months. When Rei was finally able to rescue him, Hajime stayed asleep until the smell of burned eggs in the morning triggered memories of mornings spent with his brother, and he finally woke up. After he awoke, he told the other Appli Drivers about Leviathans plan. He and Rei provided technical support for everyone in the field from a room in Kashiki Ai's book store.