Lt. Toris Vika works in DS9's engineering crew. Why on DS9? Because his level of engineering expertise lends itself to Cardassian structures and machines, leading him to be selected by O'Brien himself. Also, it's my favorite (shoosh).
He's a Bajoran, who, because of the Cardassian Occupation, did not receive medical assistance for his condition (caused by exposure to Cardassian biological weapons in utero) until adulthood. He was hidden away to keep him from being executed for being an "undesirable" laborer.
While hidden, he was smuggled books and discarded materials by his parents, and found himself proficient in and passionate about Cardassian mechanical engineering. He fashioned not weapons, but life-preserving inventions, like shields, protective masks, and air filtration systems for the underground resistance tunnels as a eenager. He even accompanied Kira Nerys over comms on a mission to shut down a Cardassian power plant and liberate a group of captured Bajoran Resistance fighters.
When the Bajoran Militia was increasing its numbers, he was offered a position as an engineering officer, but initially turned the offer down for fear of being forced into making weapons. It took the personal request of Kira Nerys for him to join, and under the strict condition that he would never be made to use or design weaponry. Later, a Kira's further suggestion, Chief O'Brien personally requested Toris for the team.
With aid from the Bajoran Republic, he's undergone spinal replacement, organ replacement, and genetic therapy for his condition, so he's in a much healthier spot now, though he requires regular treatments to keep his symptoms managed, is prone to respiratory flares, and can't go into poor air conditions. He still requires the use of mobility aids, which makes getting around DS9 particularly difficult, but he doesn't let it slow him down.
He's close friends with Kira Nerys, having known her through her work with the resistance. He's also becoming closer to his doctor, Julian Bashir. The two have bonded through their experiences with disability, and the way it affects their relationship with their respective parents. He and Garak have a unique relationship, as well, with him having been more receptive and kind to the Cardassian than was initially expected.
His main job aboard DS9 is optimization and invention for the station, creating new and efficient ways of maintenance with less chance for worker error. He also works on repairs, retrofitting, and in his spare time, development of new technology for Bajoran quality of life planetside. Many of his designs are regarded as safety gold, though he sometimes dips into grey ethics in the pursuit of efficiency, sacrificing sentient input for mechanical certainty.
One of his inventions, a rudimentary android meant for surveying and repairing damage in decompressed areas of the space station, has shown signs of sentience after coming into contact with one of the Tears of the Prophets. Terrified for his creation, as well as the implications this could have amongst the government and spiritual branches of his homeworld, Vika has chosen to hide his invention, which has named themself 'Kosst' (meaning, "to be" in Bajoran).