Who is Patrick Donohue from Stephen Coonts' Deep Black and what is their importance?

Mussa Duoar hired Patrick Donohue to search Gordon Kensworth's room after his murdered. His main assignment was to find computer disks, which were believed to be in Kensworth's possession. The disks were not there. After the search, he kills the woman and child, who were part of his cover. Duoar also hires Donohue to kill LaFoote and wants to add Ponclare, the head of DST, to the list. In the end, Duoar kills Donohue on the train.