Caste system in Kerala
The caste system in Kerala differs from that found in the rest of India. While the Indian caste system generally modelled the four-fold division of society into Brahmins, Kshatriyas, Vaishyas and Shudras, in Kerala the Nambudiri Brahmins formed the priestly class and only rarely recognised anyone else as being other than Shudra or untouchables outside the caste system entirely. Thus, the Kerala caste system was ritualised but it was not the varna model found elsewhere. Even inside Kerala, North Kerala castes are different from South Kerala. Until the recent past, they did not follow marriage alliances, citing the differences. 

Kerala's caste system was closer to feudalism than to the "Varna" system.  Nambuthiris held the vast majority of the lands.

Also, Kerala had one of the highest migration of different cultures at different times in history, due to its geographical connectivity. The migrants included Babylonians, Arabians, and Jews, and later various Europeans, as well as Sri Lankans, Buddhists monks, Jains and many more.

History
A theory presented by Pullapilly and also by Rene Barendse, who  is a Fellow of the International Institute for Asian Studies, claims that the caste system established by Nambudiri Brahmins of Kerala was in accordance with the will of Parasurama, an avatar of Vishnu.The Nambudiris had control of 64 villages and asserted that they had powers given to them by the gods, so much so that they considered even other Brahmin groups to be outside the caste hierarchy. Both writers consider this to be the traditional Nambudiri myth of origin. The Nambudiri Brahmins were at the top of the ritual caste hierarchy, outranking even the kings. Anyone who was not a Nambudiri was treated by them as an untouchable.

By the late nineteenth century, the caste system of Kerala had evolved to be the most complex to be found anywhere in India, and the exploitation of it had become considerable. Barendse explains this development: 

By this time there were over 500 groups represented in an elaborate structure of relationships and the concept of ritual pollution extended not merely to untouchability but even further, to un-approachability and even un-seeability. The system was gradually reformed to some degree, with one of those reformers, Swami Vivekananda, having observed that it represented a "mad house" of castes. The usual four-tier Hindu caste system, involving the varnas of Brahmin (priest), Kshatriya (warrior), Vaishya (business person, involved in trading, entrepreneurship and finance) and Shudra (service person), did not exist. Kshatriyas and vaishyas were rare. Kshatriya castes such as Varma as well as Vaishya castes as such a Vaniyan, Vanika, Vanika, Arya Vaishya constituted less than 2% of the total population.

The roles left empty by the absence of these two ritual ranks were taken by the Nairs as they were Kings, naduvazhis (feudal lords) and deshavazhiz.

The process of amelioration of caste distinctions by various social reform movements were overtaken by the events of 1947. With independence from Britain came the Indian constitution, and Article 15 of that document outlawed discrimination on the grounds of caste and race. Myron Weiner has said that the ideological basis for caste "... may be (almost, but not quite) moribund" and that: 

Weiner points out that despite the ideological demise: 

Although distinctions between the various communities are outlawed, the Indian governments – both at national and at regional level – do still recognise them, but this recognition is for the purpose of positive discrimination. Throughout post-independence India, including in Kerala, there exists a framework of reservation which is fluid in nature and attempts to recognise the socio-economic disparities between various castes. Depending both on local circumstances and on the changing modern socio-economic environment, castes are classified as Forward Classes (or General), Other Backward Classes, Scheduled Castes, and the Scheduled Tribes. These classifications determine what - if any - assistance a caste community receives in any given area. Formal classification lists are compiled for the latter three groups; any community which is not listed in any of those categories is, by default, a Forward Class.

Writing in the context of violence against Dalits elsewhere in India, Frontline magazine said in 2006 that:  However, Frontline goes on to note that the situation in Kerala now, is not as severe, to the extent that those seeking to research:

Untouchables
The Nambudiris had varying rules regarding the degrees of ritual pollution while interacting with people of different castes, which also included other Brahmins- such as Iyers, whose touch required a Nambudiri to bathe before resuming rituals. In a similar manner, most castes practised the principles of untouchability in their relationship with the other regional castes. Untouchability in Kerala is not restricted to Hindus, and George Mathew says that, "Technically, the Christians were outside the caste hierarchy, but in practice a system of inclusion and exclusion was developed ...". Among Christians, the established Syrian Christians also practised the rules of untouchability. In the colonial period, many lower castes were converted to Christians by the European Missionaries, but the new converts were not allowed to join the Syrian Christian community and they continued to be considered as untouchables even by the Syrian Christians. Syrian Christians derive status within the caste system from the tradition that they were elites, who were evangelised by Thomas the Apostle. Anand Amaladass says that "The Syrian Christians had inserted themselves within the Indian caste society for centuries and were regarded by the Hindus as a caste occupying a high place within their caste hierarchy." Syrian Christians followed the same rules of caste and pollution as that of Hindus and they were considered as pollution neutralisers.

The rules of untouchability were severe to begin with, and they were very strictly enforced by the time of the arrival of the Dutch East India Company in the seventeenth century. Robin Jeffrey, who is a professor specialising in the modern history and politics of India, quotes the wife of a Christian missionary, who wrote in 1860 that: 

Nonetheless, higher ranked communities did have some social responsibility for those perceived to be their inferiors: for example, they could demand forced labour but had to provide food for such labourers, and they had responsibilities in times of famine to provide their tenants both with food and with the seeds to grow it. There were also responsibilities to protect such people from the dangers of attack and other threats to their livelihood, and so it has been described by Barendse as "an intricate dialectic of rights and duties".

Sambandam

Sambandham or sambandham (literally "relationship") was a concubinage system primarily followed by the Nair and Ambalavasi castes with Namboothiri in the state of Kerala. 

During ancient times progeny out of these morganaic union where considered higher rank in caste system and had given special titles such as Nambiar in North Malabar.

All of these were matrilineal communities. The hypergamous institution of sambandam was involved in the establishment of and competition for status among these higher caste groups.The custom is no longer observed.

Demographics
Around 2003, the Government of Kerala recognised 53 Scheduled Castes, 35 Scheduled Tribes and 80 Other Backwards Classes. The 2001 Census of India recognised 68 Scheduled Castes, who comprised 9.8% of the population. They were 99.9% Hindu, with a negligible number of Sikhs and Buddhists. The Census recognised 35 Scheduled Tribes, comprising 1.14% of the population and with 93.7% being Hindus. A further 5.8% were Christian, and the remainder Muslim or "not stated".