Kevin Thomas Riley is a Navigator of Irish descent, born on the colony world of Tarsus IV. Kevin's family was murdered by Kodos the Executioner when he was a small boy.

As flamboyantly charming as any Irishman, he often annoyed the crew of the Enterprise with his singing, for he has neither the luck nor the lilt of the Irish and he CANNOT sing.

Kevin was a good friend of his helm partner, Sulu, and when Ruth Valley (see Ruth Valley) came on board, he fell head over heels in love with her. Unfortunately for Kevin, Ruth never returned the emotion, although that didn't stop her from sleeping with him, spending shore leaves with him, and generally leading him on (due mostly to her unrequited-or-so-she-thought infatuation with Spock, but that's another story). After the relationship had gone on for nearly a year, Kevin had the very bad idea to ask Ruth to marry him. Since she had, at the time, just made up her mind to confess her love to Spock and let the chips fall where they may, she turned him down. Loudly. Rudely. In public (of course, he had added bad judgment to bad judgment by proposing in public, but...). Kevin spent that night and the next day in Sulu and Jilla's cabin, getting drunk and crying on Sulu's shoulder. Sulu, trying to comfort one friend without trashing another, repeatedly told Kevin that Ruth simply wasn't the marrying kind.

Until Ruth showed up, happily announcing that she and Spock were getting married. Kevin threw some choice epithets at Ruth, who tried to apologize (really she did), and stormed out.

Not long afterwards, Kevin transferred to the U.S.S. Hood. Sulu asked Jeremy to take care of him.