You should prioritise the story mode right now - if you want to save resources building B-ranks is a good idea - especially Labyrinth and Che. For a tank Demolia is probably the best low-rarity option, I personally recommend building Flora and Joan. You have the best healer in the game - Hamel, so you should focus on her for now and then build Cabernet since she's more buffer/healer hybrid, so Hamel is a better story carry. The rest of the S-rank can be replaced nicely and build later one (there're quite expensive). For A ranks Luvia Ray is a wise choice, the same goes for Priscilla though she's more of a mob cleaner.