Light Spirit

Base spirits are generally a weak unit and will be a priority target for most of its training cycle. Its advised to keep it at the back of the map defending in combat encounters and only move in to support, when you're finishing off enemies.

Of special note is it's genetic trait light affinity which increases it's (+50%) and entire party's (+25%) exp gain. The later seems to be multiplicative with most other exp-increasing effects (including it's own +50% from light affinity) as it applies to the post-battle exp total. The combination of intelligent trait and light affinity allows you to train them very fast, making light spirits the easiest ones to evolve to lvl 20.