Danny Williams is the second-in-command within Five-O. McGarrett refers to him as "Danno." He was born and raised in Hawaii. After graduating from high school, he attended the University of Hawaii as a psychology major for one year. Then, he transferred to the University of California at Berkeley where he then switched his major to Criminology. After graduating, he returned to Hawaii and became a police officer with the Honolulu Police Department. Finally, he was picked by McGarrett to work with him at Five-O.
What are Five-O's duties?
The primary mission of Five-O is to keep Hawaii a safe place. McGarrett was chosen by the governor of Hawaii to maintain the safety of both the residents who live there and the people visiting the islands. Although virtually every case worked on are within the island of Oahu, Five-O's jurisdiction is the eight major Hawaiian islands.
What's McGarrett's background?
By the time of the first season, McGarrett's own known family is his sister, Mary Ann Whalen, and her infant son, who is dying of cancer. We're also told he is a former U.S. Navy officer, known both as a bold operator and headstrong. He also suffered from polio as a child. In an 11th season episode, McGarrett describes a bit more background. By the time of the series pilot, McGarrett's reputation is he is responsible only to "the Governor or God" and even they have trouble telling him what to do.