Is Charles the pilot of the seagull plane?
I haven't read the Koiuta light novels, but according to this guy on Chiebukuro and the Wikipedia article for Koiuta, there are strong indications that the pilot of the seagull plane is, in fact, Charles KARINO from The Princess and the Pilot. However, this is never made explicit. There is apparently a scene in the fourth volume where he and Kal-el have a conversation, but despite this, his name is never revealed. 

The Koiuta light novels are complete (no further volumes are expected to be published), so this is probably as good an answer as you're going to get.