How to customize the data retrieved by a REST call?

Question

I've enabled the REST server (plus services) and can retrieve user profile data at www.example.com/endpoint/user/123

How can I customize the data that's being retrieve? I want to not include the email address and add a couple of other fields.

When I make a login request at www.example.com/endpoint/user/login I would like to have those additional fields included as well.