Loken
Loken was a Kraylor who developed a cloaking device for his ship so that the Kraylor could defeat the Annari blockade of their planet. 

When Harry Kim came to his rescue in 2377, he asked Kim to pilot the ship, since the captain and his officers had been killed in a battle with the Annari. He deceived Kim into helping him by pretending to be a doctor who was transporting vaccines to his people. 

Kim found out later that Loken was not a doctor, but a scientist who was developing cloaking devices for the Kraylor fleet. Kim did deliver the ship back to Kraylor, but did not feel good about his first command. ()

de:Loken
Kraylor
Scientists