Mercenary
15th to 18th centuries: The Saika mercenary group of the Kii Province, Japan, played a significant role during the Siege of Ishiyama Hongan-ji that took place between August 1570 to August 1580. The Saikashuu were famed for the support of Ikkō Buddhist sect movements and greatly impeded the advance of Oda Nobunaga's forces.
Ninja were peasant farmers who learned the art of war to combat the daimyō's samurai. They were hired out by many as mercenaries to perform capture, infiltration and retrieval, and, most famously, assassinations. Ninja possibly originated around the 14th century, but were not widely known or used till the 15th century and carried on being hired till the mid 18th century. In the 16th-17th centuries, the Spanish in the Philippines employed samurai mercenaries from Japan to help control the archipelago. Abroad the wreck of one Spanish galleon, the San Diego, that sunk in Filipino waters on 14 December 1600 were found numerous tsubas, the handguards of the katanas, the distinctive swords used by the samurai.
In 1615, the Dutch invaded Ai Island with Japanese mercenaries.

19th century: Between 1850 and 1864, the Taiping Rebellion raged as the Taiping (Heavenly Peace) Army led by Hong Xiuquan, the self-proclaimed younger brother of Jesus Christ, engaged in a bloody civil war against the forces loyal to the Qing emperor. As Hong and his followers, who numbered in the millions, were hostile to Western business interests, a group of Western merchants based in Shanghai created a mercenary army known as the Ever Victorious Army. During the Taiping Rebellion, the Qing came close to losing control of China. It was common for the financially hard-pressed Qing emperors to subcontract out the business of raising armies to fight the Taiping to the loyalist provincial gentry, which formed the origins of the warlords who were to dominate China after the overthrow of the Qing in 1912.
The rank and file of the Ever-Victorious Army were Chinese, but the senior officers were Westerners. The first commander was an American adventurer, Colonel Frederick Townsend Ward. After Ward was killed in action in 1862, command was assumed by another American adventurer, Henry Andres Burgevine, but the Chinese disliked him on the account of his racism and his alcoholism. Burgevine was replaced with a British Army officer seconded to Chinese service, Colonel Charles "Chinese" Gordon.
A highly successful commander, Gordon won thirty-three battles in succession against the Taipings in 1863–1864 as he led the Ever Victorious Army down the Yangtze river valley and played a decisive role in defeating the Taipings. Through technically not a mercenary as Gordon had been assigned by the British government to lead the Ever Victorious Army, the Times of London in a leader (editorial) in August 1864 declared: "the part of the soldier of fortune is in these days very difficult to play with honour...but if ever the actions of a soldier fighting in foreign service ought to be viewed with indulgence, and even with admiration, this exceptional tribute is due to Colonel Gordon".
During the French conquest of Vietnam, their most persistent and stubborn opponents were not the Vietnamese, but rather the Chinese mercenaries of the Black Flag Army commanded by Liu Yongfu, who been hired by the Emperor Tự Đức. In 1873, the Black Flags killed the French commander, Francis Garnier, attracting much attention in France. In 1883, Captain Henri Rivière, leading another French expedition into Vietnam was also killed by the Black Flags. When the French conquest of Vietnam was finally completed in 1885, one of the peace terms were the disbandment of the Black Flag Army. Chinese flag rebels also fought in the Haw wars in Laos and northern Thailand.
Philo McGiffin served as a naval mercenary in the Sino-French War and First Sino-Japanese War.

20th century: In the Warlord Era of China, some British mercenaries like Morris "Two Gun" Cohen, and Francis Arthur "One Armed" Sutton found employ in China.
Easily the largest group of mercenaries in China were the Russian emigres who arrived after 1917 and who hired themselves out to various Chinese warlords in the 1920s. Unlike the Anglo-American mercenaries, the Russians had no home to return to nor were any foreign nations willing to accept them as refugees, causing them to have a grim, fatalistic outlook as they were trapped in what they regarded as a strange land that was as far from home as imaginable. One group of Russians wore Tartar hats and the traditional dark greycoats, and fought for Marshal Zhang Zuolin, the "Old Marshal" who ruled Manchuria. White Russian mercenaries claimed that they had considerable effectiveness against ill-trained armies of the Chinese warlords; one White Russian claimed that when he and other Russians serving Marshal Zhang they "went through the Chinese troops like a knife through butter".
Chinese forces slaughtered most of a 350 strong White Russian forces in June 1921 under Colonel Kazagrandi in the Gobi desert, with only two batches of 42 men and 35 men surrendering separately as Chinese were wiping out White Russian remnants following the Soviet Red army defeat of Ungern Sternberg, and other Buryat and White Russian remnants of Ungern-Sternberg's army were massacred by Soviet Red Army and Mongol forces.
One group of Russian mercenaries led by General Konstantin Petrovich Nechaev were dressed in the uniform of Imperial Russian Army and fought for General Zhang Zongchang, the "Dogmeat General" who ruled Shangdong province. Zhang Zongchang had Russian women as concubines. Nechaev and his men were infamous for their ruthlessness, and on one occasion in 1926, rode three armored trains through the Chinese countryside, killing everybody they met. When the Chinese peasants tore up the rails to stop Nechaev's rampage, he and his men vented their fury by sacking in an especially brutal manner the nearest town. Nechaev suffered a huge defeat at the hands of Chinese, when he and one armoured train under his command were trapped near Suichzhou in 1925. Their Chinese adversaries had pulled up the rail, and took this opportunity to massacre almost all Russian mercenaries on board the train. Nechaev managed to survive this incident, but lost a part of his leg during the bitter fighting. In 1926 Chinese warlord Sun Chuanfang inflicted bloody death tolls upon the White Russian mercenaries under Nechaev's brigade in the 65th division serving Zhang Zongchang, reducing the Russian numbers from 3,000 to only a few hundred by 1927 and the remaining Russian survivors fought in armored trains. During the Northern Expedition Chinese Nationalist forces captured an armoured train of Russian mercenaries serving Zhang Zongchang and brutalized the Russian prisoners by piercing their noses with rope and marching them in public through the streets in Shandong in 1928, described as "stout rope pierced through their noses".
Alcoholic White Russian mercenaries defeated Muslim Uyghurs in melee fighting when Uyghurs tried to take Urumqi on 21 February 1933 in the Battle of Ürümqi (1933). Wu Aitchen mentioned that 600 Uyghurs were slaughtered in a battle by White Russian mercenaries in the service of the Xinjiang clique warlord Jin Shuren. Jin Shuren would take Russian women as hostages to force their husbands to serve as his mercenaries.
Hui Muslims fought brutal battles against White Russians and Soviet Red Army Russians at the Battle of Tutung and Battle of Dawan Cheng inflicting heavy losses on the Russian forces.
Chinese forces killed many White Russian soldiers and Soviet soldiers in 1944-1946 when the White Russians of Ili and Soviet Red Army served in the Second East Turkestan Republic Ili national army during the Ili Rebellion.
During the early stages of the Second Sino-Japanese War, a number of foreign pilots served in the Chinese Air Force, most famously in the 14th Squadron, a light bombardment unit often called the International Squadron, which was briefly active in February and March 1938.

India: 

18th to 19th centuries: In the medieval period, Purbiya mercenaries from Bihar and Eastern Uttar Pradesh were a common feature in kingdoms in western and northern India. They were also later recruited by the Marathas and the British. In southern India, there is a caste/community of mercenaries in the state of Karnataka which is called Bunt. The word "bunt" itself translates to warrior or mercenary, this community later elevated itself as the rulers of the land, several powerful dynasties emerged from this community. The most notable dynasty being the Alupas of Dakshina Kannada, which reigned for 1,300 years. This community still survives and has adopted the surnames shetty, Rai, Alva, chowta etc. In Tamil Nadu, the three crowned empires used the Kongar pastro-peasantry tribes of the Kongunad region and the Kongar peasant tribes of the Erumainad region as their swordsman mercenaries, cavalry mercenaries, and as chariot soldier mercenaries, as well as personal guards. Kongars worked along with the three empires' warrior tribes such as the Kallar, Maravar, Aghamudaiyar, Parkavar, Valaiya-Mutharaiyar, and Mazhavar tribes.