Leon Sawchuk was a Human man, a Starfleet officer assigned to the command division of the Federation starship USS Enterprise under the command of Captain James T. Kirk in the 2260s.
While Sawchuk was at Starfleet Academy, he met and fell in love with Tommy Carter. They maintained a steady relationship, and eventually both served together aboard the Enterprise.
In 2269 or 2270, Carter drowned when the Enterprise was flooded by Ulum near Polymax VIII. Sawchuk was grief-stricken, but James T. Kirk gave him focus, recommending that he take the body to sickbay and then try to prevent as many others from drowning as he could. (TOS comic: "What Pain It Is to Drown")