Ibaraki refers to a minor character in the manga, a friend of Joe Shimamura who had known him from his days at juvenile hall. His other counterparts are also covered on this page.
- The Shotaro Ishinomori Character Guide vol.1 puts forward the claim that the young man in the "Birth of 009" chapter was meant to be Ibaraki, and states that he returns in the Yomi arc. Ibaraki's Yomi profile also references his early appearance. However, these claims have been disputed among the fanbase, as Ishinomori himself had never identified the first boy, and his fate has been long debated by readers (some believing him to have been intended to have died when the Black Ghost agents knocked him off a ledge).
- As the reference to the Kurihama boy being Ibaraki does not appear anywhere else, and likely due to the disputes caused by this claim, the counterpart of the inmate in Yasushi Hoshino's retelling has the name "Muramatsu" (a reference to Joe's original surname).