To unlock this job collect 9 special items called Books (locations listed below) then head into the Church to create a Ninja Star Child.
This should be doable by that point using certain maidens but the following minimum stat requirements for Ninja Star Child creation need to be met:
ATK 168
TEC 129
DEF 194
SPD 222
MAT 98
MDF 122
LUC 135
If unable to make a Ninja, continue to deepen Bonds with the Star Maidens until you can.
LOCATIONS OF BOOKS
Go to the following floors. Stand in front of any chest except those behind locked doors. Save your game and open the chest. If you do not get the book, simply reload your game until you do.
Book of Bolt
Summer Labyrinth Summer Fog 1F OR Spring Labyrinth Northstar 10F
Book of G. Bolt
Summer Labyrinth Imperial Flame 10F OR Summer Labyrinth Summer Fog 4F
Book of Lion
Autumn Labyrinth Autumn Wind 1F OR Autumn Labyrinth Twilight 8F
Book of I. Lion
Spring Labyrinth North Star 7F OR Autumn Labyrinth Autumn Wind 4F
Book of Outer
Winter Labyrinth Winter Calm 1F OR Spring Labyrinth Aries 12F
Book of Inner
Summer Labyrinth Cancer 11F OR Winter Labyrinth Winter Calm 4F
Book of Wisdom
Autumn Labyrinth Libra 15F OR Winter Labyrinth Winter Calm 5F
Book of Sun
Winter Labyrinth Capricorn 11F OR Winter Labyrinth Aquarius 12F
Book of Hidden
Spring Labyrinth Spring Clouds 4F OR Autumn Labyrinth Libra 11F