A model can have a .lua script associated with it. Scripts operate per model reference; Each "kind" of model in a scene has one virtual machine which handles all instances of that model. Pre-defined functions are called at various parts of the program for both the model reference, and for individual model instances. No individual functions are required, and they will be skipped if they are not present.
There is no function to initialize a class because this code can be placed in the main script section before functions are declared, and will only be called once.