Kōga Gengo (甲賀源吾, Gengo Kōga, February 16, 1839 - May 6, 1869) is a Japanese samurai from the Kakegawa Domain and a Bakushin, mean that a samurai who serves under direct rule lord of the Shogun. He was a boarding captain during the Boshin War and commanded the paddle corvette Kaiten where he is killed during the Battle of Miyako Bay.

Life 
Born on February 16, 1839 in the Kakegawa Domain, he is the fourth child of Hidetaka Koga, leader of the Kakegawa Clan in Totomi. In 1855, Gengo traveled to Edo and studied Rangaku with daimyo of the Sakura Domain, Guntaro Kimura. He returned to his province two years later in 1857, but in 1858, Gengo was aspired to study navigation and went back to Edo under the guidance of Kō Yatabori, who was the professor at the Gunkan Office in Tsukiji. Since 1855, the 2nd Dutch Navy Training Mission, led by Willem Huyssen van Kattendijke, came to the Nagasaki to set up a naval training center.

Gengo's older brother, Gunnosuke Gengo, along with Katsu Kaishu and Yadabori, was the only Kakegawa feudal lord to receive the first Nagasaki Dutch Navy training. However, by the time of the second period, even though he had not accepted any new people other than the Bakushin, the fact that Gengo went to Nagasaki meant that he had an informal tour of the training as a subordinate of Yadabori.

Then, Gengo studied advanced mathematics in Dutch with two of his mentors Yadabori and Arai Ikunosuke , and published a book about naval tactics. He also learned both English and French. In 1859, he became a vassal of the Shogunate and was appointed as a warship training assistant. The following year, Gengo also served as a guardsman in Yokohama. In 1861, Gengo became professor of naval training.

Life in the Shogunate Navy 
He was latter appointed as member of the warship club and surveyed the Edo Bay. The following year, the Shogunate Ogasawara Islands Inspection Team (幕府小笠原諸島視察団, Bakufu Ogasawarashotō shisatsu-dan), led by Mizuno Tadanori, boarded the Kanrin Maru to preserve the area as Japanese territory. Gengo commanded the sailing ship Senshū Maru to surveyed the area. He boarded the ship with Arai Ikunosuke as a surveyor of the sailing ship. It was difficult to cross the Pacific Ocean in winter with an old sailing ship, and the Senshū Maru was washed away and had a hard time. Although it is very short, there is a memorandum of Gengo at that time. In 1863, Gengo was ordered to survey the Osaka Bay by engaging in two round-trip voyages between Edo and Osaka to escort the young shogun, Tokugawa Iemochi. In May of that year, he was summoned to the warship group, and it became more prestigious.

In July 1863, during the Shimonoseki campaign, which began with the bombardment of the Chōshū Domain by Western forces (British Empire, United States, France and The Netherlands), Gengo headed for Shimonoseki with the messengers dispatched from the Shogunate to the Choshu while serving as the captain of the Chōyō Maru. According to a letter to his family, the messenger's mission was to dismiss the Choshu samurais because they occupied the Ogura Clan's territory, and the messenger headed for the Ogura clan territory succeeded this. The messenger who landed in Shimonoseki was killed by foreign soldiers. Gengo was forced by the Choshu Clan to borrowed his ship for the campaign, but he refused. In 1864, he was guarded according to the general's return to the sea route. In the First Chōshū expedition, he set sailed to Aki and Buzen. He became the captain of the Kishō Maru and sailed to Osaka in 1866. 

In 1867, Gengo was assigned to a fleet of small ten-person warships. That same year, the Tracey Mission of the Royal Navy arrived at the warship training center, and as the training began on November 5, he became the head of the warship training center and also served as a student councilor on November 26. The Boshin War began when Imperial forces clashed with the Bakufu troops at Toba-Fushimi.

Boshin War 
Gengo was promoted to as head of the Shogun naval fleet, but the Boshin War had already begun. After the surrender of Edo Castle, Katsu Kaishu had promised to hand over the warship to the Imperial government, but the Admiral of the Bakufu Navy Enomoto Takeaki refused to surrender, and the last remaining shogunate forces retreated including Arai Ikunosuke and Gengo. As a result of the negotiations, four warships, the Kaiyo Maru, the Kaiten Maru, the Banryu Maru, and the Chiyodagata Maru, was organized, but they were dissatisfied with the various measures taken by the new government. Enomoto decided to lead the fleet and escape after seeing the relocation of Muneya to Sunpu.

On August 19, the Shogunate fleet, whose flagship was the Kaiyo Maru, carrying former Bakufu soldiers onboard the transport ship and departed off the coast of Shinagawa. At this time, Gengo was made captain of the Kaiten Maru. The Kaiten Maru was towing the slow-moving "Kaiyo Maru", but was forced to cut the towline due to a typhoon off the Bōsō Peninsula. Due to the storm, the front and center masts were folded, leaving only the rear mast, but they arrived safely at Matsushima Bay of the formerly Sendai Domain, which was the creation of the Ōuetsu Reppan Dōmei. After that, the Kaiten Maru went north alone, and while the ship scouted the Kesen Port, they found Kaiten and captured it.

A sailing ship owned by the former Shogunate that Koga boarded when he headed for the Ogasawara Islands, but was rented to the Sendai Domain. It was decided to be returned to the Tokugawa family, but it is said that the crew had taken it and engaged in piracy.

In September, the allies of the feudal clan surrendered one after another. The Shogun fleet carrying former Shogunate troops and other army units who still wanted to fight, headed for Hokkaido. Gengo was also among them the group as the captain of Kaiten Maru.

The fleet arrived at Washinoki in the north of Hakodate on October 20, and the Battle of Hakodate began. The next day, the ship visited the Hakodate Port with thir consort ship Banryu Maru, and on the 25th, troops and sailors of the Bakufu Army landed on the island and occupied each cities from Imperial Japanese troops. On the 27th, the Akita clan warship Kaohsiung Maru, which did not know about it, arrived at the port, so Gengo had his fellow officers of both Kaiten Maru and Banryu Maru, lead a group of sailors and captured the Imperial ships, including the ironclad Kotetsu. In a battle with soldiers of the Matsumae clan (who had sworn their allegiance to the new government), Gengo supported the Bakufu Army, and when the Kaiyo Maru sailed around in Esashi, he went to rescue them, but he couldn't save it at last. After many ships where destroyed by the modernized navy, Kaiten Maru became the last flagship of the Shogunate Navy.

Killed in action and Afterwards 
In March 1869, after hearing reports of incoming Japanese forces plans to captured Hokkaido, Gengo leads a group of sailors (including Arai Ikunosuke and Hijikata Toshizō) and French Army advisors (including Jules Brunet,  Arthur Fortant, François Bouffier, Jean Marlin, and Pierre Garde). Gengo seems to have been the center of the operation because of the belief that "Let's attack aggressively rather than waiting for the enemy". The flagship Kaiten Maru, commanded by Captain Gengo, arrived at Miyako Bay along with the Banryu Maru and Kaohsiung Maru, but due to bad weather, the Banryu Maru strayed and the Kaohsiung Maru broke down. Only Kaiten Maru will enter the Battle of Miyako Bay. It had said that Gengo said in his final word before the attack, "Even my ship is enough to defeat the enemy." 

Gengo endured the inferiority and was fiercely commanding on the deck while being shot through the left foot and right arm by bullets, but as he charged at the sailors, he was shot through the head by a rifle bullet. He was 31 years old.  The ship was taken command by Arai and retreated back to Hakodate on the 26th, and Gengo;s body was buried the next day. The place of his burial was a certain temple, but it is said that it was later reburied behind the Hekketsu-hi Monument. In 1931, his nephew and adopted child, Nobumasa Kōga, built a monument in the precincts of the Daikannon Light Source Temple in Komagome, Tokyo, near the former Kakegawa clan's residence.

Character 

 Gengo's personality is describe by historians as quiet, thoughtful, and courageous man. He loved his subordinates and loved them again. It is said that there was a time when he learned to used a katana sword from an early age and decided to pursue that path.
 Before the attack on the Kotetsu, Commander Arai Ikunosuke said to Gengo, Don't be diligent and diligent in learning the work." After a while, he will come to understand it and stop, "He praised his inquisitive spirit and missed his early life.
 Admiral of the Imperial Japanese Navy Tōgō Heihachirō, who fought the Battle of Miyako Bay as third-class officer aboard the Kasuga, praised the monument, saying, "Although he is an enemy, he is also a hero who has no ability to take my prize until today, 60 years later."

Works 

 Goryokaku (NTV year-end era drama special)

Kōga Gengo : Yoshirō Ichikawa