You can store data on the user's device by using a SQLite database. You can also connect your app directly to a SQL Server database without having to use any sort of service layer.
|Use a SQLite database in a UWP app||Shows you how to use SQLite to store and retrieve data in a light-weight database on the users device. SQLite is a server-less, embedded database engine.|
|Use a SQL server database in a UWP app||Shows you how to connect directly to a SQL Server database and then store and retrieve data by using classes in the System.Data.SqlClient namespace. No service layer required.|