The Kaientai (快援隊) is operated by Sakamoto Tatsuma as a commercial company, using a big ship to traverse the universe for business opportunities.
While fighting in the Joui War, Sakamoto grew tired of watching his comrades dying in front him. He decided to try to seek a way of life that both humans and Amanto could benefit from instead of meaningless death for everyone, and settled on going out into space. Before leaving, Sakamoto asked Sakata Gintoki if he wanted to join him, but Gintoki declined the offer.
He got scammed and dumped into the ocean, later being picked up by the slave trade company Chidori, where he met Mutsu. After freeing her and the other slaves, he claimed the ships as their own, and they called it the Kaizoku.
At some point, it was later renamed to the Kaientai.
The Kaientai fleet originally consisted of five ships. The Kairinmaru, Sakamoto's personal ship, leads the fleet. Even though they are mainly a trading company, their ship is loaded with weaponry due to frequent attacks from the Harusame Space Pirates.
The ships were modified by Mutsu without Sakamoto's prior knowledge to combine the ships into a giant robot, "The Galactic Merchant Fleet Robot: Kaien", so that they could exchange business with giant aliens.
In the Rakuyou arc, the Kaientai has nine ships in total.
- Sakamato made a deal with the Renho leader Dark Vader to transport the Renho agents to spy on Earth, although Sakamoto thought he was sending pets to Earth to protect humans from an unknown disaster. Mutsu delivered Eren to Katsura Kotarou, who was later be known as Monday Elizabeth.
- In Episode 57, Mutsu made a deal with Aoki & Co. Company to sell twenty-four powerful batteries, but failed because Evil Organization stole them. After a short investigation with the help of Yorozuya, it was revealed that the company hired the organization to steal it so the CEO could file for insurance.
- During the Renho arc, Sakamoto sat down with one of the commanders in the Renho army and persuaded him, along with the rest of the Renho army, that Dark Vader had taken their freedom. He did so by pretending to sell Gintoki as a weapon and by guaranteeing that they would have an unlimited supply of ActRaiser.
- The Kaientai transformed into Kaien to defeat the Renho planet, which was being controlled by Dark Vader. It was later destroyed in the process.
- After rescuing several Kiheitai members, the Kaientai was used to bring Gintoki and the others to the planet Rakuyou to save Takasugi Shinsuke. The fleet was taken over momentarily by Uranus Hankai, who controlled the ships and members in an attempt to isolate Sakamoto. He shot the Kairinmaru's core, and only he and Mutsu joined the battle on the ground.
- Kairinmaru is likely named after the Kanrinmaru, one of Japan's first steam-powered warships.
- The original Kaientai was founded by Sakamoto Ryoma and was one of Japan's first naval fleets. It was used to ferry goods to the army. It was originally called the "Kameyama Shachū".
- The original kanji for Kaientai, 海援隊, means "Ocean Support Group". The kanji was changed to 快援隊 in Gintama, and it now means "Pleasure Support Group".