@SybillaISsoCUTE:
I use Dahlia, and I never really thought that I would like her to be cheaper to deploy despite the fact I managed to summon a second copy of her from Base last year for the -1 cost reduction, since she's a mid/late-battle unit. I believe the Black Valkyrie Clissa falls into the same category, though her AW skill does give her 2 sortie points per kill, but all of the other Skill AW Valkyries have this feature. Pegasus Rider Ridy or Esther are much more worthwhile to cost reduce, since they have an AoE attack (ranged for Ridy) and can gain sortie from flying enemy units.
Between Clissa and Elizabeth, Elizabeth would benefit from being cost reduced, so you can deploy her earlier, but I would save the Time Fairy in case you somehow managed to summon Elizabeth again from Base or SP summon for the guaranteed -1 cost. I want to cost-reduce my Witch Adelle, but I won't use any Time Fairies I gain for it until I get the -1 cost for her; there's always a chance they may make Adelle a campaign character like they did for Yuyu and Figneria.
As far as Black units go, the ones you would want to cost reduce would be ones like Aisha, Nanaly, Despia, Minerva, and so on. They're ranged units, and their cost is such that you can make one of them the first thing you deploy on a map. Characters like Heavy Armour Dine won't benefit from a Time Fairy, since she's so expensive to deploy, you're rarely going to find yourself attempting to deploy her as your opening unit.
