The Ikko-Ikki were a collection of Jodo-Shinshu Buddhists, including local nobles, monks, and peasants, who escaped from their original masters to join what was described as "a collective mind" of individuals. The Ikko-Ikki, more commonly called the Ikko, arose in the early 15th century when the revolutionary monk Rennyo revitalized the nearly dead sect of buddhism. Inspired by Rennyo's teachings the Ikko sent what they considered their leader to enter negotiations with Asakura Yoshikage, after days of negotiating Yoshikage agreed to allow all Ikko men to take up refuge in Echizen under the conditions that they fought alongside him. Due to the great number of monks in the Ikko ranks Yoshikage was quickly converted. With newfound faith the Asakura clan, now absorbed into the Ikko Ikki, lended their military strength to the Ikko, with the combined strength of Jodo-Shinshu Ronin and zealous Ikko men, the Ikko Ikki wrecked havok across the provinces and regions near Kyoto for many many years to come, crippling some clans to the point of utter destruction.
In Game: In Total War: Shogun 2 The Ikko-Ikki are the only playable "clan" that can not recruit Metsuke or Samurai due to the fanatical nature of the clan and the great opposistion to the structured military systems of Sengoku era Japan. They can however recruit Ronin units, which are stronger than Samurai units, but come at the cost of reduced numbers. Their Ashigaru units have increased numbers and morale but are less skilled than other clan units. The Ikko-Ikki specific units are just as powerful as other clan units but specialize in defensive strategies.
Ikko Ikki units also come with their own special appearance, Ikko Ashigaru units have "monk" style units in their ranks.