No I doubt that extremely Marvelous really knows what they're doing when it comes to media I think if anything it's leftovers from playtesting. Mirei and Mamori were added as extras because of the anime (game came out about half a year after the anime) it makes since considering the anime and it's ending it makes sense for them to be there in another Island. Yes lore is the reason until you come across the girls in Bhikkhuni only 1 character can be a Liberator or Extar. Mamori does have her own portrait and line for the character select screen she's visible just can't be selected with choosing Mirei first. Marvelous gives themselves time to work on projects besides they were released as DLC after the game came out. Mirei's made really well and I love playing as her a bit over powered but not the strongest in any one stat but is the only character that starts off in First Drive it shouldn't work (but besides multiplayer) she's pretty balanced she can take down bosses pretty fast which is extremely fun. She's probably one of the only characters where I like all of her Drive Breaks or specials.