I'd say Hari Seldon is the incarnation of an idea (the idea of "psychohistory"). We barely get to know him as a character, and since he is dead after 10 pages or so, he is not important for driving the plot forward. I vehemently disagree with Tolkien not being a visual author. He spends a lot of words on descriptions, sometimes to the point of exhaustion. Asimov isn't nearly as interested in surroundings. Yes, he describes lots of planets throughout the series, but they are all pretty uniform. Trantor is a grey blob, others are mostly ice or desert, and that's it. Not something that really jumps out from the page. Don't get me wrong: I like the series. I just think it is not very suited for TV. But of course, a good screenwriter might just do something great (for instance, I would have never thought that Chiang's "Story of your life" would make a great movie, but there you are).

Also, considering the prequels and sequels canon, there is R. Daneel who, being older than the Galactic Empire (and, presumably, among its architects), is certainly one of the most interesting characters in science fiction.

That goes back to a whole other series of books and small stories that many now consider part of that whole universe.

Spoiler alert?

Oh yes. This. This. This.

BTW. No spoilers please - I'm re-reading in chronological order.