Personne de confiance in French too. Homme de confiance

Apenas tome cuidado pois "man of confidence" significa "homem de confiança" e "confidence man" significa exatamente o oposto. Pegadinhas da lingua.

Letters to His Son on the Art of Becoming a Man of the World and a ...
This is certain, at least that M----t was the man of confidence with that person. Whatever be the truth of the case, there is, to be sure, ...
infomotions.com/etexts/gutenberg/dirs/3/3/5/3358/3358.htm

Col. Rowan Tells "How I Got the Message to General Garcia"
While the letters had designated me as "a man of confidence" the translator had made me a "confidence man." Hearing me through with my message from ...
www.wvculture.org/History/military/rowanandrew01.html

Letter from the Prince of Viane to his man of confidence in Palermo. Letter from the Prince of Viane to the Governor of Sicily, Juan de Moncayo ...
www.cristobalcolondeibiza.com/3eng/3eng04.htm

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Note added at 1 day1 hr (2008-06-23 18:03:14 GMT)
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Mais uns exemplos:

John C. Schumacher’s Story of WW II Shoot Down & POW Experiences ...
There were also two enlisted men who were elected by the soldiers as camp leaders and known officially as “American Man of Confidence” who could give an ...
www.rb-29.net/HTML/79SchumacherSty/13.01schum.html

A man of Duhalde will lead the Argentinean Central Bank | Emerging ...
Pignanelli is considered a man of confidence of Duhalde and its designation was also approved by Blejer, according to some sources of the organization, ...
findarticles.com/p/articles/mi_qn4174/is_20030408/ai_n12918616

Since this is a book perhaps a more idiomatic approach would be better.

Right Hand Man
Unlike yes man, the term right-hand man has positive connotations. Some[who?] assert that the term is based on the fact that most people are right-handed. It may also be based on the idea of a king having his adviser sit at his right side.

In Ancient Greece, soldiers typically fought in a formation called a phalanx, where co-operation between soldiers was vital. Each soldier carried his shield on his left side, while his right side was protected by the man standing to his right.

A biblical explanation also exists; in the King James Version of the Bible, Jesus is said to be "at the right hand of the Father(God). see Mark 16:19. Luke 22:69. Acts 2:33. Acts 5:31. Acts 7:55. Acts 7:56