Kenin refers to a vassal, a servant, etc. of high-ranked people in Japanese history. Its significance differs in ancient times and medieval times.
Kenin in ancient times
It was a class that was included in himin (humble or lowly people) according to the Ritsuryo codes. The Ritsuryo codes divided people into two major classes, ryomin (law-abiding people) and himin (humble or lowly people). Moreover, himin was divided into five classes (five lowly castes of the ritsuryo system) i.e. ryoko (imperial tomb guard), kanko (slaves to public ministries), kenin, kunuhi (government-owned slave), and shinuhi (privately-owned slave).
Among them, the kenin were privately owned by nobles, local ruling family, etc., and were treated as their property. Unlike the nuhi, they were not traded and were able to have a family; however, they were not allowed to have a family name. Although they were given only one third of kubunden (the farm land given to each farmer in the Ritsuryo system), which was given to ryomin, they were allowed to have private businesses and were not taxed.
Although their reality is not fully known because of a lack of historical records, they probably became like ryomin with the collapse of the system of the Ritsuryo codes. Though a few examples remain in the historical documents after 'the abolition of slavery,' which is thought to have occurred from the Kanpyo era to the Engi era, the class most likely disappeared shortly.
Kenin in medieval times
After the mid-Heian period, a vassal, a servant, etc. who served nobles was called kenin. In the Heian period, technical officers of shodaibu (aristocracy lower than kugyo) and samurai rank engaged master-subordinate relationships with upper nobles, such as sekkan-ke (the families that produced regents), by devoting myobu (identification), etc. They served a master with a special skill of family businesses, such as military arts and knowledge of the Ritsuryo codes, (service) and gained benefit, such as a government post, in exchange (favor). As a well-known example, TAIRA no Masakado served FUJIWARA no Tadahira as a kenin with his military art skills. That is, government officials of Imperial Court tried to improve their position by being vassals of upper nobles at the same time.
This relationship is also applies to the leaders of samurai families of noble birth. TAIRA no Tadatsune who served MINAMOTO no Yorinobu is considered a kenin of Yorinobu. Moreover, since the kamakura bakufu (feudal government headed by a shogun) was established, vassals of Kamakura-dono (i.e. the lords of Kamakura) were called gokenin (御家人) with "御" prefixed to honor the lord.