(defun isearch-yank-char ()
"Pull next character from buffer into search string."
(interactive)
(isearch-yank-string
(save-excursion
(and (not isearch-forward) isearch-other-end
(goto-char isearch-other-end))
(buffer-substring (point) (1+ (point))))))
(define-key isearch-mode-map "\C-d" 'isearch-yank-char)
白井さん作です。