Victor Hesse is an Irish terrorist, presumably from Belfast, Northern Ireland, UK. He is an exceptionally dangerous man who is on everyone's radar and on the Interpol red notice list. In the series, he is most noted for having shot and killed Steve's father, John McGarrett in cold blood, when Steve "accidentally" killed his brother, Anton Hesse.
Victor was the man responsible for the John McGarrett murder. While Steve was on assignment in South Korea. Victor had knowledge that Steve had infact taken his brother as a prisoner and issued a negotiation between them, his brother for Steve's father. The negotiations fail after an attack by a terrorist cell when Anton was able to get away from Steve but is ultimately shot and killed by him. In a rage, Victor shoots and kills John McGarrett.
When Steve returned home to O'ahu, Hawaii. He begun his search to find Victor Hesse while he still can, but to do that he had to look in the underground as Hesse would not surface. After Steve assembles the Five-0 task force, with the help of 3 other members and Sang Min, Steve was able to find and track down Victor Hesse on a cargo ship headed for international waters. After Steve and Danny storm the boat, Steve finds Victor a gun and fist fight break out with ultimately leads to Steve putting two bullets in his chest, afterwards Hesse plunged backwards into the ocean and was presumed dead.