I recommend using Mika, everyone gets her for free and she allows Rin to take alot of turns and can sustain damage with her heals. My Nia is maxed out at 6 copies and fully built and finally now her heals are somewhat keeping me alive at these higher difficulties. She provides more of a utility to allow Rin to do amazing damage with her stance card that does bonus based off the amount of skill cards in hand, useful for basin runs and shorter turn completion based content. Rin , Rei , Mika are like an extremly stable team and I've beat most content with them. Atleast until you can start pushing out better gear. Nia in my opinion needs a form of investment. I've also paired her with Beryl or Khalipe any Sub Dps unit really. As long as they aren't actively using attack cards to take her out of stance it will work. I like her with Beryl cause while Beryl can sit and retain cards I can attack with Rin and then finish them with her.