2. Nagi: If you have Saki raised already, you can spend a minimal amount of SCs for Nagi. Her passive gives her a buff against all youkai-type mobs, so she'll be useful in the Eastern maps. Comparing both her and Saki, Saki seems to be better with her assassination chance and increased ATK speed. I'll quote someone on another forum who has an experience using her in battle:
You can judge her for yourself if you want from her DMM wiki page: http://aigis.wikia.com/wiki/NagiOriginally Posted by Gabezhul
Serra: Imo, she is one of the better priestess warriors with her skill increasing her ATK and range and has heal allies in her range simultaneously. If you're looking at things in the long run, her skill AW gives her a 50% chance to increase ATK by 1.8x and nullify damage (physical and magic) as well as keeping her simultaneous healing for all allies in range. All in all, I would spend as much SCs as possible to perfect her.
Serra's wiki page for more info: http://aigis.wikia.com/wiki/Sera