Hideki Mokoto was an old Japanese pilot who bombed Pearl Harbor and killed Steve McGarrett's grandfather. He earned a medal called the Order of the Rising Sun; the medal became a piece from John McGarrett's tool box. He became wealthy after World War II and, turning over a new leaf, moved to Hawaii to help clean up the island.
He and John McGarrett became friends at a memorial service for the people who died in Pearl Harbor. Hideki later hired John McGarrett to investigate corruption in the police department and the government of Hawaii, which leads to John's investigation in to the Yakuza and Governor Jameson. Hideki lives in the forest to clean it. He sits in a wheelchair.
In Ka Hakaka Maika'i (episode), Hideki was tortured and killed by Wo Fat (according to Joe White) over information on the code name "Shelburne".