Hattori Yukio (or Yukio Hattori, if you put the first name first as in English and not last as in Japanese) is a commentator on the Iron Chef. According to the unofficial Iron Chef website (www.ironchef.com) Hattori was born in 1945. He is a food expert who runs the Hattori Nutrition College, which was founded by his grandfather. This college apparently provides the assistants who work alongside the Iron Chefs, helping them to "prepare artistic creations never tasted before", as the opening voice-over would have it.
Hattori is usually introduced as "Dr. Yukio Hattori" and sometimes called, more famililarly, "Doc", but the website claims this is a nickname adopted in recognition of his vast culinary knowledge. I don't know if he really is a doctor of something or not. He is quite dapper and intelligent-looking, though.
Hattori is something of a straight man, providing factual information about the ingredients used on the show and trivia like preparation techniques. He often commiserates with announcer Fukui Kenji (Fukui-san!), since ostensibly neither of them get to actually taste the food they see prepared before them. I'm sure if there's extra, however, they at least get a nibble.
Very occasionally, Hattori will fill in for Chairman Kaga if Kaga is otherwise occupied and can't make the show.