Rest only casts spells that inflict damage to everyone in battle so Lady will be fully healed each time it attacks. This ensures that when Lady uses White Wind, Rest will be healed as well. Rest uses Tornado or Meltdown on its first turn, and Doom on its second turn. When its HP falls below 10,240, Rest will begin using Meteor every turn, and counters attacks with either Meteor or Trine, an attack the player likely hasn't seen before that inflicts Silence and Blind on the party. The player must use Remedies to heal themselves. When Rest dies it uses Repose twice, which inflicts Instant Death and ignores immunity. Fenrir can help get around this, since Blink will block Repose.
Rest, alongside Lady, are an allusion to the Pietà, a sculpture in the Vatican depicting the recently deceased Jesus Christ post-crucifixion being cradled by his mother, the Virgin Mary.
Tetsuya Nomura's concept art of the tower of gods provides more clarity to Rest's identity. The blonde hair, scarf, and pale purple skin tone is relative to Kefka's God like final form, placing Kefka as Rest in the final phase of ascension to God. In addition to coinciding to Dante Aligheri's Divine Comedy, the tier depicts Kefka's complex process of draining the Warring Triad's power with Kefka alongside the goddess and above Doom/Fiend & Poltergeist/Demon final forms.