Bastion hates the Empire with a passion, and dreams of the day in which he will fight as an honourable Kingdom soldier against the Empire. But, as you could probably tell by now, Bastion is destined for greater things.
Bastion's ATAC of choice is the Alba ATAC, which is a special model used for the king's personal guard. Later on, he will acquire either the Ultragunner ATAC or the Tic-Tac ATAC, depending on the choices you make. Make sure you balance his stats equally, and you will be rewarded in the end with powerful attacks and Dormant abilities.