Shiro is Kouji's younger brother. Most of the time, Shiro's role is to add some comic relief to the plot. He remains in Japan when Kouji departs at the beginning of Great Mazinger. His character acquires a bit more depth in Great Mazinger, as he becomes acquainted with his father Kenzo, who was previously thought to be dead. Toward the end of the series, Shiro finally bonds with Kenzo, who died shortly thereafter.
He even gets to pilot his own robot in Great Mazinger, Robot Junior.
In the American dub of Mazinger Z, TranZor Z, Shiro's name was changed to "Toad." Shiro is voiced by Kazuko Sawada in the original version and by Mona Marshall in English (he is one of the few Mazinger characters whose English voice actor in TranZor Z has been confirmed).