When the Amanto arrived and the Joui war started, Gengai tried to make machines to help the war but his son Hiraga Saburou 平賀 三郎 disgusted with the thought of him making killing machine participated in the Joui War, he saw his son's decapitated head and was in shock ever since.
He wears a pair of industrial grade eye-goggles, is almost bald, has a distinctive grey beard and some missing front teeth.
He makes ugly and crude machines, and his belief is that 'The machines of men are armour, cannons and Caterpillars' 「男のからくりは鎧・大砲・キャタピラ」.
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 (からくり堂 karakuri dou), 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.
In the Lotus Chapter he helped repair Tama by re-using No.305's body.
In the Tama's Virus Infection Chapter, he helped analyze the problem of Tama's pixelated look, sending the Yorozuya into her body to fight the viruses.
In the Kabuki-cho arc, there are rumours that the Okama and Yakuza are going to tear down Otose shop. He and his army of robots appeared to help in the battle against the men of Doromizu Jirochou.
In chapter 318 he repaired Sa-chan's glasses, and in chapter 321, it was discovered that he actually turned the glasses into some kind of mass destruction weapon.
In this arc Hiraga Gengai help to recover Tama due to serious damage caused by Kintoki, by that, Gengai also helps Gintoki to recover the memories of the Kabuki District.
Yorozuya tried to acquire Gengai's help to fix the time machine. However, that almost always ended up with Gengai or Madao's death. Eventually, they seek other ways to repair the time machine.
Gengai rebuilds the legendary Neo Armstrong Cyclone Jet Armstrong Cannon. With the assistance of Kintoki and Tama, nanomachines are being fired to stop all engines and electronics. He is then captured by the Yato clan, and is shortly rescued by Kamui and his army.
- He is based on Hiraga Gennai (平賀源内) who was a pharmacologist, physician, author, painter, and inventor of the Edo period.
- He has a strange fixation with shouyu aka soy sauce, and modified Kagura's umbrella and Gintoki's bokutou aka wooden katana to squirt soy sauce.
- Takeshi Aono, Gengai's original seiyu, died in 2012. Since then, he is now voiced by Bin Shimada.
- He bears a significant resemblance to the Engineer from Team Fortress 2, as they are both short, stout engineering geniuses who are bald and wear goggles.
- "Machines are meant to serve a purpose to people. That's certainly true. But if you two had said you didn't want to, I don't think I would have minded. Even if you don't serve a purpose, even if you're pieces of junk, if I'm with you, my friends, I'm ready to drop dead with you."
- ↑ 'Caterpillars' here refer not to the brand name but to the generic term of caterpillar tanks, construction equipment and certain other off-road vehicles, i.e. continuous tracks vehicles.