I'll tell you what nickname I gave to the character of Owari no Seraph! It's not an official nickname, but I think it's true for each character.
List of Nicknames
Ferid= Ferd, because this nickname remember me in German word for horse. For me, the face of him is similar to a horse. There is even a MEME from Ferid with Horseface in the Pinterest.
Krul= Lolita Queen, because she looks so similar with a lolita girl in Anime Artworks.
Lacus= Lacy, because this name is better than Lacus. Who wants to have the lake as his first name?
Guren= Roy Mustang Doppelganger, because he looks like the Roy Mustang from Fullmetal Archenmist.
Mikaela= Mirror of Naghi, because he looks confusingly similar. Both looks young so kawaii!
René= Punk Guy, because, he hairstyle looks so punkerish.
Norito= Playboy, because he like playboy magazine. (2.season)
Rigr Stafford alias Saito= Sai, because he*s black eyes remind me on Sai from Naruto.
Thats is all Charakter of Owari no Seraph they I gave nicknames.