As part of Account(TMF666) entity, there are no "Characteristic" sub-resource as part of Open API model whereas other entities have the same, what is the reason that same is not included as part of API structure or Entity model for account?
In general, we discourage the use of unbounded characteristics in the API model, since it results in weak contracts (it becomes a random bit-bucket where you can add anything you like).
If you need to extend the model, there is a well-defined methodology as documented in TMF630 design guidelines.
We use characteristics primarily when the characteristics are defined at design/configuration time and are inherently dynamic - such as product catalog/product inventory, trouble ticket specification/trouble ticket, etc. In these cases there is a specification entity that defines what the characteristics look like, and an instantiation entity that has the actual characteristics and values, but only according to the specification definition.
It is true that there are entities in the model that have unconstrained characteristics, but these are there historically and were not removed.
Hope it helps