LogFAQs > #967379381

Rino 6 star upgrade should in September. She is literally the next 6 star release. If you haven't gotten her UE to 140, you can save 50 shards for the 6 star upgrade. She's still mostly useful for PvP, but that's still because AOE is more useful in PvP than PvE. 6!Rino is a good multi-target attacker in general.

Monika is slowly losing usefulness. Her UE is important for her, but she's still only good on short fights (fights less than 50s IIRC, anything over that Kokkoro is better even before 6 stars, and S!Saren outclasses them both regardless as a speed booster). Tamaki is still useful when her niche is needed (kill magic user). Nanaka is also useful now that she's finally in the BA shop on global. Nanaka's a decent mage, with an AOE MDef shred, so worth building for a budget mage.

For BA, I'd say it's a toss-up between Tamaki and Nanaka for the second pick. After those two would be Monika or Hiyori, though Monika has a long time before she gets her 6 star, while Hiyori should get 6 star in about half a year.

For PA, it would be Kaori or Anna after you finish Miyako. Suzuna's outclassed by Shiori, Tsumugi is a disruptor with limited use in PvE, and Hatsune is a niche physical killer. If you're trying to build a mage team, Anna with UE might have priority over Kaori, but otherwise Kaori is probably better to work on first.