Tactician:

A eudemon can become a Tactician after character level 20. You'll get 1 more BP and gain more EXP according to its quality.

Requirements:

character level 20

Click on the 'Appoint' button and all available candidates will appear.

Tips:

If your eudemon is below 1 star, only warrioratkdef and mageatkdef can be appointed as Tactician.