There is one name Ludwig calls Feliciano that Feliciano doesn't know, because he doesn't say it aloud.
Feliciano knows liebling (most common in the mornings, muttered against an unshaven jaw or given with a cup of hot coffee) and Schatzi and Süßer (usually during the day, phone calls and quick lunches and once quite memorably in front of the Chancellor before Ludwig could catch himself), Felichen and just plain Feliciano and 'Ciano (for public), and Spatzi or Mausi (usually Ludwig's being silly when he says that, but he does mean it—Feliciano is small, even if he denies it and stands on tiptoes in official photographs, and quick and inquisitive), and Schön (mostly in bed or kind of tipsy, and honestly Ludwig feels like he should say it more often)—all these, Feliciano knows Ludwig calls him, and he chuckles and calls Ludwig tesoro, amato, bello in return.
He doesn't know, because Ludwig doesn't say it aloud, Engel.
Ludwig does call him Engel in his head, during the quiet soft moments he never thought someone like him would ever know, because it is because of Feliciano that he knows them and he would never want another.
Because Feliciano had stayed by his side at the worst, the lowest he'd ever fallen and that had been so unimaginably low and without Feliciano there—Ludwig knows he would have never been able to pull through without Feliciano there.
Because Feliciano is beautiful in his doe-eyed, lean, runner's-body way, and terrible when he absolutely has to be, and strange and kind and not unknowable but unfathomable (and he's missing the holy fire, but he's warm).
Because Feliciano saved him from becoming cold and withdrawn, saved him from cracking apart and shattering, took Ludwig's hands and guided him and let him cry but held him through it—
Because when it first hit Ludwig, really hit him that when Feliciano said I love you, I'm in love with you and returned his kisses that he meant it, one day while he was just putting away the butter, it was like the time Gilbert had taken him to a cathedral in France when he was small and they had been walking down the nave and then the sun had come out and shone through the stained glass and everything was light and color, stunning—
Because sometimes it hits Ludwig that the strange, soft-edged, bright-burning love he carries inside himself for Feliciano cannot be entirely earthly, not when Feliciano reciprocates it in the same way, not when the way Feliciano nuzzles into Ludwig's chest in his sleep or hums when he cooks makes his heart constrict sometimes with sheer how—
And Feliciano is liebling, Schatzi, Süßer, Felichen, Spatzi, Mausi, Schön, but most of all he is Engel, and maybe one day Ludwig will even tell him so.
liebling: darling; Schatzi: treasure; Süßer: sweetheart/sweetie; Felichen: diminutive of Feli; Spatzi: little sparrow; Mausi: mouse; Schön: beautiful; Engel: angel (schutzengel: guardian angel)
tesoro: treasure; amato: beloved; bello: handsome