For healers, you have three candidates which are all rather effective. Liana is costly, but would have the highest raw power and arguably the best ability out of the three.
Liana will heal 900 HP to all of your units, with the exception of those who can't be healed, when she is deployed. Of course her skill is rather niche, although her Awakening Skill has a nice addition of healing all units on the field, except for those who can't be healed, 40 HP every .5 seconds during her skill.
Chydis has the weakest ability, just a +10 to MR. Normally that would be useful, but Chydis has too little HP for it to really be effective except for against those that deal low magic damage already. However, her skill and Awakening Skill have excellent usage and utility in their boost to power and range.
Iris' ability is a mini-version of Liana's, 500 instead of 900 HP. Her Awakening Skill is powerful, but it gets weaker with each use. It starts off with 2.1 times power, but the second time it's used it has 1.8 times power, and the third and subsequent times it has 1.5 times power.
Now for a few other useful classes and units to awaken, Katie and Aria are nice since one or the other will be used often and the power up to themselves as well as their global 2% to Defense and Offense respectively, while not much, could mean the difference between taking/dealing chip damage or not.
HAs get a massive boost to their HP, between 900 and 1000, making them solid sponges for the stronger opponents in the future, as well as upping their stats and block count to 4. Maribel will get a 15% chance to negate any attack, whether physical or magical, while Bernice will boost the defense of all HAs by 5% just by being in the party. Do note that this awakening is a little more costly since HAs have their cost increased by 2 upon awakening, however the sheer stat boost is worth it.
Finally, if you decide to awaken a mage, Garania would likely be a better option since her ability is a 7% boost to the attack of all mages, and would boost her up to be a little more powerful than Odette (although an awakened Odette still has higher base stats than awakened Garania and would be even more powerful with an awakened Garania in the party, but I digress). Odette just gets a +10 to her MR, which isn't that bad since with her skill she can be a pretty decent magic lightning rod, but is still rather niche since turning her into a magic lightning rod could draw lethal physical range as well.
