In Storage Service, collection is like table in RDBMS database. Ideally collection should be an Entity/Noun of your system. For example, User can be a collection in which you can store user information like his name, address etc in JSON format.
In your case it is better to create User/UserProfile collection and store username and his related information inside.
Once JSON is stored you can apply find/update method using stored information. See this for complete details.