I just finished reading "Darwin, His Daughter, and Human Evolution" by Randall Keynes, which is a sweet book, full of family lore (he's a descendant).  What comes across strongly in the book is how sensitive Darwin was, and devoted, as a husband and father.  I've read lots of Darwin, but never really got that sense of him before,  and thought (even more) that he was someone one would like to spend some time with.  (He did vomit a lot, so that might be an occasional drawback). So the thread title includes "nice" -- rather than "I'd like to meet X so I could tell him off" or such. 
My initial candidates would be Darwin, Plutarch (based on his letter to his wife), Montaigne (his inherent decency shows through), and (though he wasn't an author) the painter, Camille Pissarro.  There is a fine short piece on him in a recent review of an exhibition which talks about how devoted he was to everyone around him, and how they reciprocated in various ways (Cezanne, who was notoriously bad tempered, in his final show labelled himself: Paul Cezanne, pupil of Pissarro).