Healing Light is a spell that can heal one or all allies.
Charlotte, Duran, and Kevin can learn Healing Light. How much you heal from the spell depends on your Spirit parameter. It costs 3 MP to use. The effect can also be simulated with the Poto Oil, Lord's Proof, and Holy Water Vial items.
Since Charlotte is the healer of the game, she learns Healing Light (single ally) after the player obtains the spirit Wisp and increases her Spirit parameter a few times. Regardless of class change, Charlotte always learns the spell's upgraded version for all allies.
Duran will learn Healing Light (one ally) when you class change the first time toward the Light side (Knight). He will only learn Healing Light (all allies) if you send him toward the Dark side (Lord) during his second class change.
Kevin will also learn Healing Light (one ally) when you turn him to the Light side (Monk). He will learn Healing Light (all allies) when you send him to Dark (Warrior Monk) during his second class change.