Endo was not only both popular and highly regarded in Japan: his books had many admirers in the English-speaking world, including Graham Greene, Francis King, Penelope Lively, John Updike, Angus Wilson. And, for all the seriousness of his work, he had an additional reputation in Japan as an entertainer, even a bit of a clown, running a chorus of singers who proclaimed themselves to be the worst in Japan.
He was very good company, and astonishingly generous. In the late 1980s, he set up a trust with rich Catholic friends (in particular Kenzo Kogi, who runs a travel agency) in order to organise and finance the exchange of writers, artists, and musicians. In September- December 1989 I was the beneficiary of such a grant, and towards the end of my time in Japan Endo invited me to join him and a few friends on a tour of Nagasaki and other southern sites, including several significant ones in the history of early Japanese Christianity. Though weak in health (as Kirkup points out), Endo never seemed to let this drag him down; he was lively, learned, affable, and witty.
I think it extremely unlikely that any "grievous disappointment at not receiving the Nobel Prize had contributed to a sudden decline in his already unstable health". Endo had too much sense of balance, and too much sense of humour, to have allowed any such thing.