Spectral Force 3

To recruit Trick Blue, you need to complete the mission "Retrieve Rare Minerals".

Mission requirements: The Meimai Knights are destroyed and complete the mission "Supply Line in the Desert".

Recruitment requirements: Locate and grab the item Trick Blue is looking for.

Once you have unlocked the mission, complete it by opening chests until you find the item that Trick Blue is looking for. You'll know you've found the right one when the mission ends. When given a choice, agree to give Trick Blue the item and he will join your party.

The achievement will unlock after you have saved your game.

EDIT: Thanks to Albino Bob 325 for helping to clarify the language in the Recruitment requirements. Appropriate changes have been made above.