Early years & Starfleet Academy
Jenan was born on Trill and was well educated. Unjoined, Jenan applied to Starfleet in search of fame and fortune. His experience was tarnished; after earning high marks at the Academy he was passed over for multiple promotions. He eventually retreated back to his homeworld to apply to the Symbiosis Commission. He went through the trials required of a prospective host but did not meet the expectations. While aboard a starship with his field docent, an incident occurred where the captain of the ship was disabled. His Starfleet training took over and he led the crew, saving the ship from certain disaster. This action won his field docent's vote and he was put forward to be a host.
After the Joining process, he felt a need to try Starfleet again. The experience shared from the Gage symbiont pushed him further than he’d been before, allowing him to quickly climb the ranks until he was made Commander and appointed First Officer of the USS Colfax. During one mission the ship was attacked by Romulan pirates. Gage led a boarding party to disable the Romulan’s Valdore from the inside. The Colfax was destroyed - along with its captain and most of the crew - but Gage’s team was able to take control of the Romulan vessel and the pirates were imprisoned. He was promoted to captain and given the USS Mariner as his first command. ("Teaser 1")
During his first run in Starfleet, Jenan acted as a mentor for some cadets. Among them was Amelia Chou, an aspiring tactical officer. They grew especially close, but Gage wasn’t satisfied with the direction his career was going so he left both Starfleet and Chou to return home to Trill. Years later, Chou was posted to Mariner by request of Rear Admiral Bryant, against Gage’s objections. Gage would go out of his way to emphasize his professional role when interacting with Chou, presumably to keep their working relationship in check. ("Shakedown")
Tara Leilani was Gage’s right hand. She advised him regarding difficult decisions, using her advanced mental abilities as needed. Gage shared many command roles with her in almost a sort of partnership.
The Gage symbiont had 5 previous hosts. One died at the hands of Romulans, with the symbiont surviving long enough for another host. A later host’s husband was killed by Romulans. Each of these deaths has tainted the symbiont's view on the galaxy. Combined with the attack on the Colfax, this leaves Gage with a special distaste for Romulans. ("Teaser 1", "Shakedown")
Character biography on Mariner’s official site