Spectral Force 3

To recruit Little Snow, you need to complete the mission "Snow Queen".

EDIT: Thanks to xDartz xNVx for the following information:
I recently discovered that you must complete the mission "A Wave of Ghosts" (Langeais's recruitment mission) in order to be able to unlock Snow Queen.

Mission requirements: Attain the rank of Licensed and the Overlord Army is destroyed.

Recruitment requirements: Complete the mission.

Once you have unlocked the mission, simply complete it to have Little Snow join your party. Little Snow will be present from the beginning, but will not move or help you out. Fortunately, the enemies seem to have little interest in her either. At first, there will be almost no enemies on the map. Once you lay them low, a wave of angels will spawn in on both sides of the map, and then once more.

The achievement will unlock after you have saved your game.