Taro Aso, Secretary-General of Japan's ruling coalition and a close ally of Prime Minister Shinzo Abe, is widely seen as the frontrunner to replace Abe after he announced on Wednesday he would resign.
Here are five facts on Taro Aso.
Aso, 66, served as foreign minister in Abe's first cabinet, before taking the ruling Liberal Democratic Party (LDP)'s number-two post as Secretary-General in an August 27 reshuffle. He has also served as minister for economic planning and for posts and telecommunications.
Aso came in a distant second to Abe last year, in his second bid for leadership of the LDP. Coming from a venerable political family, Aso has been open about his desire for the top job. His grandfather, then-prime minister Shigeru Yoshida, negotiated the peace treaty ending World War Two. Aso's father-in-law was also a prime minister and his sister is married to a cousin of Emperor Akihito.
Aso shares Abe's goal of a bigger global security role for Japan. Last October, after becoming foreign minister, Aso said there was nothing wrong with discussing whether Japan, the only country to suffer an atomic bombing, should possess nuclear weapons. But he has also said that he would stay away from Tokyo's Yasukuni Shrine, seen by many in Asia as a symbol of Japan's past militarism, if elected prime minister.
A rarity among Japan's mostly staid politicians, Aso appeals to fellow fans of "manga" comics, and can work a crowd with amusing patter. He recently authored two books, one of which -- "Tremendous Japan", about Japan's "tremendous" potential -- has become a best-seller.
His brash manner has provoked controversy. Aso was recently forced to apologise over a flippant remark about Alzheimer's disease, and he stirred anger in the two Koreas in 2003 for remarks seen as praising Japan's 1919-1945 colonisation of the peninsula. Earlier this year, he criticised US policy in Iraq and said Japanese with their "yellow faces" would be more successful at Middle East diplomacy than "blond, blue-eyed Westerners" since Japan had never exploited the region.