I think Heinlein's character (singular) was strong, convincing, and fascinating. The fact that the entire dramatis personae of his complete oeuvre was played by that same character, over and over, in drag, age makeup, and blackface where necessary, is just another fascinating tidbit. Oh, nearly forgot the other character: a strawman "bad guy" who pops up once in a while, having all the character of a broom with a face painted on it and a black hat perched atop. But he doesn't count.
If you doubt this assessment, re-read "The Number of the Beast", which should confirm it nicely for you.
And even with all that said, I re-read Heinlein with pleasure just to get more into the head of that one character, Admiral Lazarus Valentine Podkayne Stone Smith.