So like a UK domestic flight? ID still needed which is likely to be a passport, national identity card or driving license.
In which case a pax with missing ID might be a problem, albeit solvable.
You need an ID or passport to enter the aircraft in Spain. You do not need it in Germany.
You can get past security and to the gate without ID everywhere in Europe, though, if your destination is in the Schengen area.
This combination makes it dangerous to miss your return flight just at the gate as one does not notice the missing ID on the flight ex Germany (happened to me in France and was resolved by my airline ID card after a lot of discussions only)