The real life Denys George Finch Hatton was British, but when you have an actor like Robert Redford on hand, historical fact needs to give way to sheer movie star power.
Hence: no British accent.
Denys is pretty much every woman's dream, at least on the surface. Tall, handsome, modest, and totally sure of himself, he strides off all manly-like into the bush, then comes back to talk Mozart and philosophy under the stars.
What's not to love?
Karen sure thinks so, which is why she lets nature take its course once she kicks her hubby to the curb. He shows her the freedom she always wanted in Africa, and—unlike a lot of men at the time—he respects her opinions and intelligence. And he's pretty happy to tell her that when the subject comes up.
As you may have suspected, however, it's not all roses: the man just ain't gonna commit.
DENYS: I won't be closer to you and I won't love you more because of a piece of paper.
He likes his freedom, and being able to just walk out into the savanna whenever he wants. He likes escaping. He likes getting away.
Attractive loners are something of a Sydney Pollack specialty, and Denys fits the bill perfectly. Pollack told a reporter:
Let's say you have a guy who's a loner—he's terrifically intelligent, he has a sense of humor but he also has a dark side. Women find him very, very attractive but he wants more out of life. Well, if you cast Bob Redford, you don't have to write as hard. (Source)
Denys just can't provide the one thing that Karen really wants (besides, you know, not getting syphilis again): commitment.
KAREN: What's wrong with marriage, anyway?
DENYS: Have you ever seen one you admire?
KAREN: Yes, I have, many. Belfield's for one.
DENYS: He sent her home for the rains in 1910. Didn't tell her they were over until 1913.
That's where their romance ultimately hits the skids. You can see their chemistry, feel the heat they have for each other and think, "hey, they can really make a go of it out here." But that means you need to know the other person is going to be there for you, not out getting gored by rhinos.
He's honest and straightforward about what he wants, but that isn't what she needs. They both know it, but that doesn't make it hurt any less. They hash it out over the campfire one night:
KAREN: When you go away... you don't always go on safari, do you? Just want to be away.
DENYS: It's not meant to hurt you.
KAREN: It does.
DENYS: I'm with you because I choose to be with you. I don't want to live someone else's idea of how to live. Don't ask me to do that. I don't want to find out one day that I'm at the end of someone else's life.
But this isn't a movie with a happy ending, and so Denys—beautiful, hunky Denys, who lives like some sainted cowboy—gets to die in a plane crash and leave his lady love alone. His need for freedom is part of what she finds so attractive about him, but it's what drives them apart—forever, ultimately.
Ironic, isn't it?
Funny thing is, Karen's aware of it. When she visits his grave after his death, she tells us:
KAREN: He was not ours. He was not mine.
It's sad, but it's also who this guy was: the best man she ever knew and the one she could never have.