point man

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A soldier who is assigned to a position some distance ahead of a patrol as a lookout.
• noun A man who has a crucial, often hazardous role in the forefront of an enterprise.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Mil.) the lead soldier in a foot patrol under combat conditions.
• noun a person who takes a conspicuous public position in proposing a new idea or initiating a new policy, who may become a target of criticism for those opposed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military In combat, the soldier who takes point; the soldier who assumes the first and most exposed position in a combat military formation; the lead soldier/unit advancing through hostile or unsecured territory.
• noun By extension, a most trusted assistant or associate; the person to whom one would most rely upon.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who is the forefront of an important enterprise
• noun a soldier who goes ahead of a patrol