Man, Riki's got annoying voice acting but it's just not possible for a westerner to produce those high-pitched squealy mascot character voices the Nopon were designed for. They should have had him voiced by a girl at least though.
Now that I'm up to 6 characters I'm trying all the off-characters as my active party. Would be painful to encounter Mechon without Shulk in the party but if I do I can just re-configure after a wipe. Melia is interesting to play, she has a completely different combat system than other characters. First you have to summon elements that provide a status boost aura while summoned and then you can use her special skill (which doesn't require a charge to use) to launch them at enemies. Seems you can have up to three elements summoned and each type can be used multiple times (but since each summon is on a separate cooldown timer it's faster to summon three different ones). Dunban as a main character seems suicidal, he's pretty frail and attracts enemy aggro (even has skills to draw aggro, as opposed to Shulk who has a skill to AVOID aggro) so he gets killed quickly. Supposedly high AGI stats can make him survivable (the game throws you a bunch of help pages for every character but only logs them in the tutorial menu so you have to look them up before using the character) but I haven't played much since re-outfitting him for agility instead of armor.
Seems inactive characters get reduced XP (at least there's a skill for having them get full XP on one of the characters). Can't be by much though, I think I have them all at the same level (or maybe that's because XP decays exponentially with your level so an underleveled character would gain exponentially more XP than the others, bringing them up to speed quickly).