Lumas was a Caatati. Most of his species, natives of the Delta Quadrant, had been assimilated by the Borg sometime before the year 2374. He and a few thousand survivors managed to escape the calamity on thirty ships, but with nothing, and were forced to roam the quadrant as beggars, starving and destitute.
In 2374, he and his fellow survivors encountered the USS Voyager and asked its commanding officer, Captain Kathryn Janeway, for supplies. Janeway was able to spare some supplies, but was unable to give all that he wanted. After coming aboard the Voyager and attempting to get more by trying to make Janeway feel guilty for not giving them more, even though she could not, he saw Seven of Nine in a corridor. On finding out she was a former Borg drone, he tried to attack her, and had to be restrained by Tuvok.
When Voyager's warp core went critical and had to be ejected, the Caatati stole it when it did not explode. Lumas used their possession of it, along with surrounding the Voyager with nearly all of the thirty ships they had, to attempt to force Captain Janeway to surrender all of Voyager's supplies and weapons to them. He also demanded Seven of Nine, whom many of them wanted to make sport of in revenge for what the Borg did to them. The crisis was resolved when Seven built an energy matrix for them, which produced thorium isotopes. Caatati technology ran on thorium; thus, with this matrix, and schematics on how to make more of them, they could thus create all the supplies they needed. (VOY: "Day of Honor")