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Marcassin has faster casting speed (or faster recharge speed, I forgot) compared to Oliver, though that's the only advantage he has over him. Personally, I always alternate my party members after every level. Sure, Marcassin doesn't have a unique skillset, but when it gets down to it, I never had to capture familiars after I got the familiars I wanted and I never had to steal from enemies if I didn't need their items, so Esther and Swaine never felt like permanent additions to my party like some people feel they are.