Who is Major Dennis Michaelson from Subterranean and what is their importance?

Major Dennis Michaelson is the leader of the Navy Seals. The Seals accompany the expedition team into the caverns. Michaelson's little brother was one of the first members of the expedition to disappear, and Michaelson continually helps Ashley and Ben as they travel deeper into the caving system. He also stands by Ashley after she is blamed for the murder of Mo'amba.