Callie and Marie are both great, but Marie won the final Splatfest.
The squid sisters, the Inkopolis pop icons, consists of Callie and Marie. Calamari is the plural form of the Italian word for squid and is the culinary name for squid.
The character Crusty Sean is a play on the word "Crustaecean."
Almost every inkling in the game (except Callie and Marie) is around the age of 14 due to them gaining the ability to transform around said age.
During the early stages of development the inklings were originally planned to be rabbits, however the developers could not find a reason why rabbits were shooting ink so instead they chose squids.