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You can kind of switch the roles of Cal and Ria depending on the episode — and they do also have a mentor/student relationship going on. Same probably goes for Foster and Loker, because he is definitely The Chick in terms of 'not good at anything much' and 'there to look pretty', but Foster is definitely the most empathetic character.
The six principal Oceanic 815 survivors are arguably the overall 5-man band, throughout the entire series, with several minor bands being formed during various periods. Bear in mind that everyone's roles switch frequently, but this is the general layout: