The first thing you learn as an A-Rank Agency Bounty Hunter is to never go out of your Snakebird's sight.
Before Rank A, you don't get paired with a Snakebird, because you don't get sent on missions dangerous enough to require a Snakebird. They only have so many Snakebirds, they don't lay eggs very often, but there aren't many Bounty Hunters able to survive enough service in the Agency to be invited to take the qualifying exams, never mind actually have a hope of passing them. After all that, your bird has to not reject you. It was touch and go with Garian and Jack. They hated each other on sight, treated each other with mutual suspicion. Garian believed every single rumour he had heard about the birds, how they were the results of illegal genetic experiments, how they were wired up to a database with more information on everyone than the Government, how they only used a fragment of their psychic abilities when they saved their partners from scrapes, even healed them and augmented their strength, and when they were really pushed into a corner, they could rewind time. How it was impossible to kill one. How the Agency was secretly run by one. How they only really partnered with the high-ranking Hunters so the birds could keep potential threats to them under surveillance. How the only two remaining S-Rank Hunters in existence didn't have Snakebirds after the other two, the Lansky twins, had somehow managed to kill their Snakebird partners before they were apprehended by the entire rest of the flock and frozen in liquid carbon (don't get him started on the rumours about the carbon freeze ghosts, their living nightmares that infect the waking world and all the other reports from that island of people going there for perfectly legitimate business and never coming back).
Or there was the other rumour about S-Rank Hunters, of course, that they underwent a genetic modification procedure to BECOME Snakebirds! After all, almost nobody had ever seen a real S-Rank Hunter, so nobody really had proof that it was just a rumour.
For his part, Jack had also heard a fair few interesting rumours about Garian - that he had been involved in some genetic experiments of his own, back when he was an orphan and Rajeen's messed-up laws meant that the Agency could do pretty much what they wanted with him. That it was no coincidence that the only two beings in existence who could capture and replicate the DNA of others were Garian and Bilan, or that he was the only person who had been able to read the creature's movements so intuitively that he had managed to bring the monster down (although in that case, how had he managed to kill his partner, huh?).
Then Garian had asked how Jack even knew so much about Bilan in the first place seeing as the monster was supposed to be such a big mystery to the Rajeen government that it was assumed to be a Lavian secret weapon.
Then Jack had accused Garian of defending Lavian because he was an enemy sympathiser.
At which point Garian had called Jack a paranoid nationalist nutjob.
After an hour of this increasingly heated argument that now involved yelling, screeching, flying feathers, a sword going through an expensive computer and an illegal psychic power alarm going off, the scientists were puzzled to find that swordsman and bird were now completely synchronised as partners. They shrugged and told both of them that they were working double shifts to pay for the extensive damage to the room.
And it wasn't until that fateful night on the boat where Garian stepped into the captain's cabin to fight Bilan, then went off radar, only to reappear on the island with a child's blood on his sword, insisting it was Bilan while ranting about judgment and the afterlife and infinite false second chances and the door to paradise being always in sight but just out of reach, that Jack ever left his partner's side.