As far as men go, Dannie is a lot better than Richard. He's logical, well-spoken, and romantic, plus he respects Therese. He talks to her like she's an equal, not a little girl, and he's honest with her and up front about his emotions. Frankly, he's better than Carol, too, but there's one problem with Dannie: his chromosomes.
Dannie is barely in the book, slightly more than his brother Phil, whom we first meet when he helps get Therese a job and then proceeds to be completely pointless to the story. Dannie, however, provides an important contrast to both Richard and Carol. Therese really likes Dannie when he doles out his wisdom, including gems like "Life is an exact science on its own terms, it's just a matter of finding them and defining them. What doesn't make any sense to you?" (10.133), or "I think friendships are the result of certain needs that can be completely hidden from both people, sometimes hidden forever" (10.121). Wise, this one is.
Unlike Richard, whom Therese will probably never speak to again, we think Dannie and Therese will continue to be good friends. Plus, Dannie helps Therese realize that she is not going through a Sapphic phase—she is definitely attracted to women because if an amazing man like Dannie does nothing for her, no man will. Only Carol will do.