John F. Kennedy had one. So did Simone de Beauvoir. Machiavelli was one. So was Cardinal Richelieu.History tells us of the great conquerors, the leaders who changed the course of civilization as we once knew it. We learn about the genius artists and the wunderkind athletes and musicians whose singular talents could only have come from within. In many cases, these figures made the history books on their own merits – solo artists in the annals of achievement. But so often, a deeper examination reveals a close adviser just offstage, someone giving counsel, delivering criticism and providing the occasional dose of reality.Do you have a trusted adviser? Do you serve as one?Recently, New York City’s Mayor Bloomberg lost one of his closest counselors, Press Secretary Stu Loeser. While not the type of character one might associate as being in billionaire Bloomberg’s inner circle (a New York Times article described him as, “A singular presence in his fedoras, chunky glasses and disheveled suits”), Loeser was nonetheless the Mayor’s staunchest supporter in front of the press, and his most honest and direct adviser behind closed doors.Far from serving as a “Yes Man” to a leader not overly familiar with the word “no,” the Times article describes heated conversations between the two men, and a respect that was borne from Loeser’s ability to shoot straight with the Mayor. Said Bloomberg: “I’m not happy he’s going. He can identify problems before they happen, and make sure they don’t happen.”In a recent profile of living rock legend Bruce Springsteen, the New Yorker acknowledges another lesser known, but highly influential behind-the-scenes operator: former rock critic Jon Landau, who crossed the critical divide and became Springsteen’s closest friend, adviser and manager after writing one of the most storied lines in pop culture criticism history: “I saw rock and roll future and its name is Bruce Springsteen.” Landau went on to help curate Springsteen’s evolving worldview that moved beyond the Jersey turnpike and onto a fully American stage framed by John Steinbeck characters and John Ford landscapes.Another long-time friend, E-Street Band guitarist Steve Van Zandt also serves as a brutally honest partner in crime, once directly criticizing Springsteen for some narcissistic lyrics he penned for the song “Ain’t Got You.” “We had one of our biggest fights of our lives,” Van Zandt says. But something resonated, and Springsteen’s writing sharpened in the wake of an honest assessment not often delivered to a rock god at the height of his powers.If you’re a leader, who is your closest adviser? Do you encourage him or her to tell you like it is? Do you respect their honesty and directness? If you work with leaders, what are you doing to become a trusted adviser? Do you raise your hand when those around you seem reluctant to offer constructive criticism?Not everyone will make the history books, but we can all make a difference in our organizations by offering counsel to others, or by simply accepting it.