Norah is Our Heros older sister and will be found at the start of chapter 5. No special mission is needed to unlock her but she does have some pretty good skills you may like to take advantage of, eg.
Petrify: a powerful slash with a 20% chance of paralyzingly a targeted dark creature for 4 seconds
Lull: reduce the speed of a targeted dark creature by 50% for 4 turns
Defend: reduce damage from attacks by 40% and increase speed on the timeline by 70% the following turn.
These are all her defult class skills and can be upgraded and made better throughout the game. Think of her like the hope and rubella like the vanille of child of light.