I beg to disagree that the nature or essence of currency is consensus, at least not until Bitcoin came. It may be so in a small and primitive community but it cannot be in an advanced and large society.
The fiat currency is made up of a lot of policies, decisions, changes, impositions, etc which are not out of consensus. Of course it could be argued that it is hard to gather the opinions and ideas of everybody and that the very reason why there is a government is that it would act and decide on behalf of the larger population.
However, this is not really the case about fiat. The decisions of the few persons who decide about the direction of the currency are not really reflective of the people's voices and sentiment. So everything is merely imposed on the citizenry whether they like or not.
The essence of money is value it.
The currency is considered to be the main exercise of three functions: value scale, exchange medium, value storage. Now people think that the core of the currency is trust. Since trust, it slowly formed a consensus.
I will just add basically the point of the currency is that it is acceptable to everyone, so we can use it flexibly. on the other hand, there must be approval from the government so that all people must be able to comply.