In Mazinkaiser OVA, Juzo Kabuto was retconned into surviving without Koji or anyone's knowledge, and worked in another secret underground laboratory where he finished the final and more powerful version of Great Mazinger and then his ultimate creation, Mazinkaiser. He later appeared to Koji via hologram, telling him about Mazinkaiser. Whether he was still alive, and if so, where he currently was, is unknown.
