Rest only casts spells that affect everyone in battle so Lady will be fully healed each time he attacks. This ensures that when Lady uses White Wind, Rest will be healed as well. Rest uses Tornado or Meltdown on his first turn, and Doom on his second turn. When his HP falls below 10,240, he 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. When Rest dies he uses Repose twice, which inflicts Instant Death and ignores immunity.
The party can steal a second Ultima Weapon from Rest. If the player is playing the Advance or mobile port, this is highly recommended as two Ultima Weapons are a powerful combination in Dragons' Den.