Bastion unit SST Laboratories Siege Automaton E54 was created in an unnamed omnium and had been left in domain in the forest near Eichenwalde for at least twenty years. It is assumed that the last objective of Bastion E54 was attacking the city of Stuttgart during a time where the Omnic Crisis was active. For a short time before its reactivation, its right shoulder was the location for Ganymede's nest.
In The Last Bastion, Bastion reactivated and had its first meeting with Ganymede. Although the program led it to its former objective, Bastion strayed away from the route to fulfill its curiosity about nature, following Ganymede around. After being alerted by the sound of a nearby woodpecker, mistaking it for the sound of gunshots, Bastion changed into Sentry mode as its combat programming took over and destroyed part of a forest, mowing down multiple trees and scaring its bird companion away. Dejected by the destruction it had caused, Bastion continued marching toward its objective dismally.
After exiting the forest, Bastion came across a fallen Bastion unit in a meadow and attempted to view the fallen units memory through means of optical data transfer, reviewing this data E54 was returned to a moment in time of an explosive battle between multiple omnics and human soldiers of the German Defense Force and the Crusader legion. After viewing the fallen's memory Bastion's face diode turns red and as it recalls its purpose, Bastion stands and charges its weapon preparing to continue the conflict towards the nearby city, its combat programming affirmed. Ganymede flew to the omnic again, offering the twig in its beak and stopping it in its tracks. After a moment of confliction between the past and the present, Bastion's combat program shut off (as seen with its diode turning from red to blue) as it willingly made the choice to return to its friend versus destruction it once wrought. Overjoyed by seeing its friend again, Ganymede and Bastion went back together to the forest to live a hopefully peaceful existence with nature.
While not much is known about Bastion's personality, it's clear that it displays consideration and curiosity towards the natural world and its creatures, as opposed to its fellow units which were determined to attack every human in sight during the Omnic Crisis. As an omnic, Bastion prefers non-populated areas due to people that may see it as a threat. Thoroughly, Bastion is shown to be a gentle and harmless robot that also possesses some fondness for Ganymede.
As tender as it may be, Bastion still retains its core combat programming that is capable of effectively turning it into a mindless killing machine from the former days. The programming will take over its processor and aim to eliminate any threat whenever Bastion comes in contact with anything relating to the Crisis, or hears any sounds that may indicate a threat nearby.