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:
Quote Originally Posted by Gabezhul
Nagi is best considered to be a duelist Angel who also deals a bit of melee support damage outside of skill activation. Her SAW in particular turns her into a pretty decent magic lightning rod for the duration by doubling her health and granting her 30 MR, but unfortunately it can only be used once per battle, and otherwise it is pretty lame (a 3400/800/700 Black unit is pretty lackluster in the current meta). Overall she is not a terrible unit, and if you need a melee support unit for single target DPS instead of anti-rush purposes she might fit the bill, just don't expect her to turn the game on its head.
You can judge her for yourself if you want from her DMM wiki page: http://aigis.wikia.com/wiki/Nagi

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