Creation and development
Hiroya Oku developed Kurono's character with the goal of having manga readers identify with him. He was also created to "make a synthesis of the basic conditions for a work of entertainment". Oku originally created Kurono as a character who would fit in a college comedy series. To make him the Gantz character readers would identify with, the author gave him traits such as his sense of insecurity, fears and other complexes. Before the manga started serialization, Oku told his assistants that with Kurono's exception, all the major characters from the series would die in order to surprise readers with plot twists. As the series continued, Oku felt that Kurono was his favorite character though he stated it might have been because he was the main character. In the making of the live-action adaptations, he felt that Kazunari Ninomiya fit the image of Kurono well. In another interview, Oku said Masaru Kato was his favorite character based on his heroic traits in contrast to Kurono who he felt was a more flawed character with conflicting thoughts.
Gantz: O director Yasushi Kawamura said that as a result of Kato replacing Kurono as the protagonist of the film, he felt unsure of the audience's reaction even if they had not read the original manga. For the live-action series, director Shinsuke Sato said that Ninomiya was perfectly cast for the role, that he visually resembled the manga character and represented the current audience. According to Sato, Kurono goes through some "very subtle changes of the heart" in the manga and Ninomiya could portray those particular qualities.
Kurono's Japanese voice actors were Daisuke Namikawa in the anime and Yūki Kaji in Gantz: O. Namikawa stated that his first impression of Kurono was that of an attractive person. However, once reading his lines while interacting with other characters, Namikawa found Kurono cynical and later stated that Kurono was bold due to how he often spoke his mind. Because such people are rare, he felt that viewers would sympathize with Kurono. Due to the frequency of recurring characters dying in the anime, Namikawa joked to fellow actors if they would stop working together and feared Kurono might also die in the plot. He admitted wishing that Kurono would obtain a power to get through difficult situations, and enjoyed seeing Kurono surviving the action scenes in each story arc. Once doing so, Namikawa felt the character became a hero as well as a "great person". Despite liking his character, Namikawa felt it was challenging to voice Kurono.
Christopher Ayres voices Kurono in the English dub of the anime.
Kurono's English voice actors were Christopher Ayres in the anime series and Lucien Dodge in Gantz: O. Ayres had previously worked with Matt Greenfield of ADV Films dubbing for Megazone 23. Ayres auditioned for Kurono and for Kato, and was given the former role the following day. He originally thought he was going to voice Kato, but the director felt he was more fitting for voicing Kurono, whom Ayres felt was a "jerk". Initially, most of Ayres's friends told him Kurono was a "jerk and an ass" which left an sudden impression on him. Greenfield did not want Ayres to know of his character online, instead wishing to learn of Kurono while working on the English dub. Upon working in the Gantz dub, he stated "I've always been drawn to anti-heroes and characters with an edge, and Kei is definitely both. I love the fact that he's flawed and makes mistakes [...] Perfect people are so much more boring to play as an actor." He enjoyed working as the character to the point of relating with him.