Cassie's a waitress who hopes to serve more than beer to Ennis Del Mar. Yet if she asked Ennis, "Coffee, tea, or me?" he'd probably say, "Just water."
Divorced from Alma, Ennis is too hung up on Jack to move on with anyone else, even though Cassie's very tolerant of Ennis's silent ways.
Cassie's presence complicates Ennis's sexuality. He's interested in her…just not as much as he is interested in Jack. But by this point in his life, Ennis no longer wants to live a lie. He can't live the truth either, and move in with Jack, so he lives alone. He ignores Cassie as a result, and she says to him in their final conversation,
CASSIE: I don't get you, Ennis Del Mar.
Don't worry, Cassie. No one does.