Hiraga Gengai is Edo's greatest mechanic and inventor. He has helped Sakata Gintoki and the Yorozuya on several occasions.
He makes ugly and crude machines, and his belief is that 'The machines of men are armour, cannons and caterpillars.'
Gengai has a bad habit of modifying everything he has at hand, forgetting to tell the items' users important instructions on how to utilize them until it's too late. This usually results in the items' users getting into trouble.
Hiraga Gengai was first introduced in episode 17. Seemingly as a normal mechanic, the Yorozuya later found out that his only son, Hiraga Saburou had lost his life in the Joui wars. His only son's death left him with a deep emotional scar and Takasugi Shinsuke took advantage of that and used him to attack a festival in which the Shogun would be attending. However, with the help of the Yorozuya and the Shinsengumi, the attack was stopped and Gengai persuaded that revenge against the Bakufu was not the right course of action.
Presently, he is the owner-proprietor of Machine Hall, a machine-repair workshop and inventor's office, and he frequently has to repair Gintoki's scooter. Even so, he is still a fugitive from the law, as seen from his wanted poster at the end of Episode 17, and as stated by Gintoki in the beginning of episode 67.