IReadOnlyList<Windows.System.User> users = await Windows.System.User.FindAllAsync();

var current = users.Where(u => u.AuthenticationStatus == Windows.System.UserAuthenticationStatus.LocallyAuthenticated &&
                               u.Type == Windows.System.UserType.LocalUser).FirstOrDefault();

// user may have username
var data = await current.GetPropertyAsync(KnownUserProperties.AccountName);
string displayName = (string)data;

// or may be authenticated using hotmail/outlook
if (string.IsNullOrEmpty(displayName))
{
    string firstName = (string)await current.GetPropertyAsync(KnownUserProperties.FirstName);
    string lastName = (string)await current.GetPropertyAsync(KnownUserProperties.LastName);
    displayName = string.Format("{0} {1}", firstName, lastName);
}