Aya

Aya's niche puts her here but she is one of the few units that could actually be argued down with the reason that Aya's first 2 turns are generally "dead turns" for her meaning that she will pretty much always use her personal skill and wait. Additionally, if she gets accidentally put into combat, she generally has to repeat these dead turns. That said, once she is stacked out, she can debuff a ton of enemies super hard (Poison Nova), kill a threatening enemy easily (Hellfire/Meteor), create powerful allies (Animate), or chip a ton of enemies pretty significantly (Astral Reckoning/The Breaker).

Aya is a really unique character in the fact that she can absorb ally's debuffs with her Demonic Call and turn it into buffs for her with her passive, Demon's Blood. Aya can get these buff stacks with Demonic Call either maxed out (or really high) in 2-3 turns. Since these stacks cap out at +30, you want to get there (or as close to the practical application) as soon as possible - the more mastery, the more stacks she gains. Additionally, you could change the glyphs around if it isn't putting you where you want. Entering combat with her is a last resort - don't do it unless you have no other options.

Aya's best class is Cosmician but she can be effective at any class besides Relic Knight in her regular class tree.

Cosmician - Due to the +15 mastery at full HP and the high mastery growth Aya gains her buff stacks the fastest in this class. She is a really strong debuffer with Poison Nova and we want that to debuff as hard and as much range as possible as poison will reduce all stats by x amount. Additionally, she REALLY likes the Inferno Ring with this setup, buffing both the damage of Hellfire and Meteor. Alternative Rings she can run: Lava, Mystic, Heir.

Necromancer - Necromancer is an interesting class on her. Realistically, you will be debuffing foes with Poison Nova and having your allies clean up, so positioning is really key to this if you want to get mana back. With Inferno Ring, she can clean up a foe with Hellfire. Animate is really strong and can be driven down to a pretty sustainable spam and even with the low efficiency, the clones can tank a hit or two. Remember to play around with Demonic Call's glyphs to get +30 (or as close to it as practical) by turn 2. 

Loremaster - Necromancer is a boring but efficient class for her. Because she's generally just going to be your debuff bot, she's generally just going to be spamming Poison Nova with the occasional Hellfire. The Breaker is kind of an interesting skill on her and the AoE is massive. There's some strategy hitting your allies, but Risk vs Reward. Rousing History is probably never worth it, though does cost slightly less mana than Poison Nova if unglyphed and can be glyphed to apply poison to proc Blackflame passive. Really though, if you're using this class, you're using it for the Making History passive.