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 did not know what love was — asking Tegan "Love? What is love?" He was only interested in her for the feelings she gave off.
In the end, Marriner was sent away to the Eternals' realm by her willpower; before he vanished along with Striker, he held his hand out imploringly to Tegan, begging her to help him. (TV: Enlightenment)