What is Belle's secret?

Question

The final pages of Death of a Ghost describe Mrs. Lefcadio looking at a self-portrait of her husband, and much is made of how different it is from the famous Sargent portrait of the man. And then she turns it over:

Written across the back in the painter’s enormous hand was a single phrase:
'Your secret, Belle darling.'
The old lady returned to the portrait. She touched her lips with her forefinger and pressed it on the painted mouth.
'Oh, Johnnie,' she said sadly. 'Such a lot of trouble, my dear. Such a lot of trouble.'

What is the secret John Lefcadio was reminding Belle of, and which of the novel's troubles did it cause or represent?

Answer 1

Let's look at the full quote describing Lafcadio's self-portrait here:

It showed the same face which smiled so proudly from Sargent, but there was a great difference.

John Lafcadio's famous beard was here only suggested, and the line of his chin, a little receding, was viciously drawn in. The lips were smiling, their sensuous fullness overmphasized. The flowing locks were shown a little thin and the high cheekbones caricatured.

The eyes were laughing, or at least one of them laughed. The other was completely hidden in a grotesque wink.

It was cruel and revealing, the face of a man who was, if half genius, also half buffoon.

Belle Turned it over. Written across the back in the painter's enormous hand was a single phrase:

Your secret, Belle darling.

So John Lafcadio has painted a very unflattering self-portrait which pictures him as a buffoon. In fact, he and Belle both knew that he was in fact, partly a buffoon, and this is the secret. This aspect of his personality is what led indirectly to most of the novel's troubles — although I won't go into detail here to keep this answer relatively spoiler-free.