Ronin function as a vastly superior version of the Doppelsoldner, their attacks dealing AoE damage and having high multipliers against cavalry. The Ronin's siege damage is also extremely high, as they match those of an Imperial Doppelsoldner (without any upgrades).
When it comes to close combat, Ronin are almost invincible, capable of slaying any unit that comes in their path, including the strongest cavalry units such as the Mahout Lancers and other heavy infantry in mere seconds. Ronins can defeat any forms of infantry and cavalry relatively easily. Due to having very high melee damage, they can become nightmares to cavalry and infantry based armies. One of the ways to counter Ronin is to have Artillery protected in the back with Heavy Infantry or to mix Spies and Ninjas in your army as they have large bonuses against mercenaries.
Ronin can only be sent by the Ottomans, Portuguese, and Spanish at the Industrial Age through the Asian Mercenary Army card, which consists of 7 Manchu and 5 Ronin. However, the Portuguese can hire them in the Fortress Age with the Passage to Nipon card, which sends 5 Ronin. In the original Age of Empires III, the Ronin, along with the Manchu, is the rarest mercenary due to the fact that it can only be hired by three civilizations mentioned above. In The WarChiefs, all European civilizations have the Dance Hall card which enables Ronin to be trained from the Saloon.
The Hatamoto Samurai is identical to the Ronin, but are easier to access since they can be trained from the Japanese Consulate if Japanese Isolation is chosen. They start with +10% hit points and attack than the regular Ronin, and are automatically upgraded in every Age starting in the Fortress Age (see here for the exact values).
There is a unit in the Editor called the "Repentant Ronin", which suggests that Ronins were going to be available for Asian civilizations as well.
There is an occasional glitch in the Editor where Ronins can attack, but will be unable to deal damage to other units, including other Ronin.
The Hatamoto Samurai's large bonus against cavalry is most likely a reference to the Nodachi/Odachi, a large two-handed sword intended to counter cavalry by using its size and mass to cut the horses' legs or slice into the horses' breasts.
"Ronin were samurai who had no lord and, by extension, an uncertain role in feudal Japan. A famous group of ronin were samurai loyal to the daimyo Asano, who was disgraced and forced by the strictures of honor to commit ritual suicide, or seppuku. His death made ronin out of 47 of his samurai, who set out to avenge their master's dishonor. They killed the daimyo Kira who had lured their master into disgrace and, while admired by the people and considered heroes, these Ronin were eventually sentenced to death. They were given permission to follow their daimyo into the afterlife by commiting seppuku, rather than being executed as criminals.
Ronin, as samurai, fought with a wide array of weapons: katana, nodachi, kama, jutte, and their bare hands - though the katana is iconically associated with samurai. They wore elaborate armor, enameled and richly decorated."
Edit
Infantry Breastplate (+10%)
Thin Red Line (+20%, British only)
Corselet (+25%, Spanish only)
Cree Tanning (+5%)
Maya Cotton Armor (+20%)
Navajo Weaving (+5%)
Customized Merc Weapons (+30%, John Black's Mercenaries only)
Carib Kasiri Beer (+10%)
Mapuche Tactics (+50% siege attack)
Zapotec Cult of the Dead (+20%)
Yoga (+5%)
Master Lessons (+10%)
Military Drummers (+10%)
Tilly's Discipline (+20%, Germans only)
Incan Road-building (+20%)
Apache Endurance (+5%)
Town Watch (+2)
Standing Army (-25%)
Incan Chasquis Messengers (-25%)
Mapuche Ad-mapu (-10% coin cost)
Coffee Trade (-10% speed, Dutch only)
