Hiraga Saburou (平賀 三郎) was Hiraga Gengai's son. He died during the Joui wars, fighting alongside fellow samurai. He was a mechanic.
He has black hair
Before the Joui war, he and his father enjoyed inventing and making machines. Gengai eventually became the best inventor in Edo. During the Joui war, Gengai made more robots to aid the Joui in the effort against the Amanto. Saburou, who missed the enjoyment they both had in making machines, was unhappy with the use of his machines in war, and volunteered to join the Joui war instead. He joined the Kiheitai, as noted by Takasugi.