Marriner was an Eternal who served as first mate on the Shadow under Captain Striker when they competed for Enlightenment.
All the Eternals desired to experience existence through "Ephemeral" beings; Marriner became particularly infatuated with Tegan Jovanka and her thoughts and emotions. She believed that he was experiencing romantic feelings for her, but Marriner was only interested in her for the feelings she gave off. In the end, he was sent away by her willpower. (TV: Enlightenment)