@Random Wanderer: One is a loud, brash, outspoken American that plays Sensha-do for fun (therefore, treating it as nothing but a game), and has no problem expressing her frustrations even through swearing. The other is a calm and reserved Brit who treats Sensha-do as serious business and expresses herself through vague proverbs. They are basically the opposites, plus the whole US-vs-Britain thing. They also have some things in common, both value fair play and are very friendly, ready to go out of their way to support Ooarai simply because they like the girls.
What is interesting, however, is that although I say "American" and "British", both of them are Japanese (Kay is from Sasebo, while Darjeeling comes from Yokohama), it is just that their respective schools have different themes (US and Britain), so I always wondered how much of their personalities is actually theirs and how much is it playing the role.
I mean, all the GuP characters are Japanese who basically go by the Hippo Team's logic of 'soul names' appropriate for their theme (Matsumoto Riko, for example, is simply known as Erwin, because that is her thing), or they go by nicknames (Anzai Chiyomi ---> Anchovy), so neither Kay nor Darjeeling are actually named Kay and Darjeeling.
The only real foreigner is Pravda's Klara, a transfer student from Novosibirsk who spends much of the time speaking fluent Russian, and while Nonna is able to hold a conversation (stands to reason a Russian-themed school will teach Russian), Katyusha amusingly does not understand a word of it and insists they all speak Japanese.
So I just wonder, how much of Darjeeling and Kay's compatibility (in the eyes of the fans) is actually down to their personalities, and how much is it to them playing their respective schools' themes?