The key is "are never going to be used outside of the class"
Structure doesn't matter when the implementation remains private. You're free to change/modify it however you want because nobody will depend on the structure.
Public members/interfaces OTOH, do matter. When you expose a member publically/internally, you should have an explicit reason for doing so because somebody may/will depend on it in the future. That's the dividing line between application and API development. In APIs access control becomes very important because any member/interface that people depend on is one that can't be changed/removed without breaking somebody's code.
In essence:
your private parts are nobody's business but your own
Part 1
This is a good design question. You are correct in detecting code smell concerning getters and setters. They generally indicate a design problem exposing the implementation details of your object.
Try to think in terms of what your objects should do - Tell, Don't Ask:
Your first problem may be that you are trying to design "data classes". Rather than worry about the data (implementation details), think about the functionality. Again, what should your objects do? In your case, what do you want to do with character styles? Who (software-wise) cares about character styles? What do they need to do?
Hopefully that gets you started. Test driven development helps with these kinds of design problems. It forces you to think in terms of function not data.
On the contrary, if all you need is a data container, write a C-style struct class and go to town. I wouldn't recommend that, maintaining it will be a $@#%#.
Good luck!
Part 2
What you want is either a simple data class - skip the getters and setters completely, or you want to create a platform independent abstraction that places a facade (Facade Pattern) in front of the rendering and style setting. It simply provides an interface for setting styles and rendering. Your platform specific implementation does the dirty work (using NSTextView in your example).
The benefit of the simple data class is that it is initially simple to write. Its drawback is that you will have a hard time avoiding a giant tangle of if-else statements. You will also be lacking a clear place to make you platform specific rendering calls that use the styles. As the system's complexity grows, you may find it more diffcult to decide where implementation details go.
The facade is a more abstracted approach. The benefits are that it is more flexible and can be reused if you decide to port to another platform. Its drawbacks are more upfront development time.
The facade's public interface will provide what you need to set and remove styles as well as initiate rendering when the time comes.
The details of how you want to set styles are up to you. Use what ever system feels best. Simple getters and setters or a generic set and get that uses a dictionary internally works too (see boosts ptree if you are using C++). You could even take all (or default) styles at construction time. You could decide to not even expose mutators at that point. Your call. Perhaps you decide it is important to data drive the styles you support and use a configuration plus factory system (We can add more detail later if that is important to you). In fact different implementatios of the facade could provide different ways of approaching the problem. You could prototype a few and choose what works best.
The platform specific implementation of you facade abstraction will use the platform specific rendering system (NSTextView in your case) and the styles you have set to make the appropriate calls to the system. Simply inject the platform specific classes at construction (Dependency Injection), implement your render() method and you should be good to go.
Part 3
If your system design allows, you could take all styles for a particular element at construction time. This could allow you to avoid getters and setters completely if you chose to make your element immutable. Your would then have a simple, clean and possibly immutable abstraction in front of your character styles system. Immutable state generally leads to fewer bugs but does require you to operate under the premise that you cannot change things willy-nilly.
Taking this a step further, a configuration file might define the different style setups you have. Again this would require prior knowledge of what styles you are setting (similar to constructing with the styles above). Give the type of style you are seeking, say "heading", you might fetch the configuration for headings which specifies a larger, bold font.
These are just some ideas off the top of my head. Without further requirements gathering and use cases it will be tough to get more specific.
Hope that helps. Good luck!
Best Answer
You should seriously consider using the observer pattern and, rather than calling an Update method which iterates through a list of objects that need an Update, you simply call an event to which all of your objects are subscribed for as long as they are valid to be updated?