Caysin Bog was a humanoid male from the high-gravity world of Teres Lutha Minor. He was blown apart in an insurgency strike on the moon Jedha, only to be surgically pieced back together by a surgeon calling himself "Roofoo." Bog was present in Jedha City in 0 BBY and witnessed an attack by the Partisans on an Imperial patrol shortly before the city was destroyed by the Galactic Empire's Death Star superweapon.
Bog managed to avoid being killed by the Death Star and following the Battle of Yavin, he was working as a mercenary with his lover Tam Posla. Around 1 ABY, the pair accepted a job from the Son-tuul Pride crime syndicate to travel to the planet Skako Minor under the command of Doctor Chelli Aphra. Once on the world, the mercenary group was led into an Imperial trap by the Skakoan Hallio Bas; however, Aphra managed to kill Bas and the Imperials with an exploding tooka.
The group then made their way inside the lab, where they were attacked by another group of Imperials led by Magna Tolvan. The arrival of the Imperials triggered the awakening of a chthonic worm god, that then attacked Bog and his allies. After the mercenary droideka Dek-Nil killed the beast by collapsing the roof of the lab, Bog and the others looted the lab and then returned to their shuttle.
After discovering their memories they were searching for was stored in Hivebase-1, Aphra led the mercenaries to the rebel flight school. They attempted to sway General Hera Syndulla about their "plan", but she locked them away. Although they managed to break out, Bog and the others stayed in their cells to win Syndulla's trust and she became willing to listen to their plan. Bog, though, released an array of pistols from his neck compartment, allowing the mercenary Sister Six to stun the rebels. They then kidnapped Syndulla, much to Posla's disgust, and flew away.
On the way to Hivebase-1, Aphra fixed Bog's diagnostics, while implanting a contingency order into him as a means of prompting Posla to fight for them. They arrived at Hivebase-1 pretending to be trading Syndulla with the Imperials there. Aphra, however, lured in the flight school to distract the defenses and they infiltrated the complex undetected. Although their presence was unknown at first, it was given away while inspecting they were inspecting frozen prototypes, which were then defrosted. Aphra activated Bog's contingency order to have him run into the firing line so Posla could attack the prototypes. Bog died in the process.