Ms Renho, a Taiwanese-Japanese mother of two, has been chosen to lead Japan's largest opposition party, the Democratic Party (DP), in internal elections.

She thus becomes the de facto opposition leader, poised to take control of the government if Prime Minister Shinzo Abe's Liberal Democratic Party (LDP) were to fall from power again.

The 48-year-old's feat is particularly remarkable in a largely homogeneous society that frequently scores lowly in global gender equality indices and where "haafu" (people of mixed origin) are frequently seen as "not Japanese enough".

More than that, she won last week's party election despite a citizenship fiasco. The former Cabinet minister who goes by only one name, Renho - the Japanese pronunciation of her Chinese given name Lien-fang - admitted to holding dual nationality just days before the polls, following media speculation about her nationality.

She had claimed earlier in her campaign that she had renounced her Taiwanese citizenship when she became a naturalised Japanese citizen at 17 years old.

At her press conference retracting this claim, she apologised for "causing various sorts of confusion because of my inaccurate memory".

The incident was "nothing but a sorry state of affairs", said conservative daily Yomiuri Shimbun in a scathing editorial. "(She) has such little understanding of what it means to be a politician that her quality as one will be put into question," it said.

Japan's laws state that anyone with dual citizenship who chooses to be a Japanese citizen is "duty bound" to drop the other nationality, although there are no penalties for any failure to do so.

Ms Renho has since applied to renounce her Taiwanese citizenship.

Despite the brouhaha, she easily beat her two competitors for the top DP post: former foreign minister Seiji Maehara, 54, and former finance ministry bureaucrat Yuichiro Tamaki, 47.

The telegenic politician is aligned to a majority of her party members and is more popular than her rivals, based on her Diet track record. Also, when the nationality issue blew up, many members had already voted in early voting.

She becomes Japan's second female opposition leader, after the late Takako Doi who headed the then-largest opposition party, the Socialist Party, three decades ago.

Ms Renho, whose pristine white suits stand in stark contrast to the dark colour palette favoured by Japan's male leaders, has a sizeable undertaking ahead of her.

The former swimsuit model and television newscaster has been tasked with repairing the DP's reputation as an incompetent party, acquired after three years of botched rule from 2009 to 2012.

The economy declined during its reign, which was also marked by the shambolic handling of the Fukushima nuclear crisis. And then there were the policy flip-flops, unkept promises and infighting.

Ms Renho is acutely aware of the need to rebuild the party's fortunes, telling a news conference last week that the DP has some "soul-searching" to do. The approval rating of the DP, known as the Democratic Party of Japan (DPJ) until it merged with a smaller opposition party this March, remains in the doldrums.

It scored a paltry 7 per cent in media opinion polls this month, while the LDP got 40 per cent. This is despite divided opinion over the LDP's "Abenomics" economic policy and its intention to revise Japan's post-war pacifist Constitution.

Ms Renho said after her victory: "We are up against the juggernaut of the ruling coalition and (Mr Abe's) highly popular administration. But we won't just criticise the ruling power. Rather, we'll propose actual counter-policies so we can become the party of people's choice."

She has slammed Abenomics as a policy that favours the better-off, leaving behind many people including single-parent families and students with college loans. In its place, she is urging greater investment in students and their education, as well as higher salaries for nursery teachers.

But she is prepared to look past party stripes on national security issues such as North Korea's nuclear threat: "There have been repeated outrageous acts by North Korea, and on this point, there is no opposition party. We are united in our stance."

Ms Renho, who is married to freelance journalist Nobuyuki Murata and has 19-year-old twins, first rose to prominence in the 1980s as the poster girl for car audio products maker Clarion. Later, as a newscaster, she reported on events such as the Kobe earthquake in 1995.

When the DPJ was in power, she held posts such as Minister for Government Revitalisation, earning plaudits and the nickname "hissatsu" (shoot to kill) for her grilling of bureaucrats over potentially wasteful government projects.

Professor Koichi Nakano, who teaches political science in Tokyo's Sophia University, told The Straits Times that Ms Renho has "occasionally shown brilliance in holding the government accountable as a sharp parliamentary debater".

But her mixed background could make her susceptible as a "target of dirty attacks from the right wing".

The third-term Upper House lawmaker has popular appeal - she consistently scores the highest number of votes in the six-seat Tokyo constituency in polls and has 400,000 followers on her Twitter account.

After being declared DP's new leader, Ms Renho said: "I'll stand at the forefront of our fight to rebuild the party to win an election again."

But political watcher Yoichi Kato of think-tank Rebuild Japan Initiative Foundation noted that she won the DP election with the backing of Diet members who are satisfied so long as the DP can "stop the leading coalition from winning a two-thirds majority". In doing so, they have no qualms about entering "ad hoc coalitions" with other opposition parties.

"How she can take her party beyond this wall, without upsetting her support base, is the most crucial but difficult question that she faces," he told The Straits Times.

"So far, she has not squarely addressed this big question. She won the presidency by trying to please as many as possible, instead of taking positions on tough questions."

Dr Nakano also said that Ms Renho "lacks policy substance", adding that she "has the tendency to utter ill-advised words and expressions too lightly".

Besides the citizenship fiasco, she was also criticised during the hustings for calling her predecessor Katsuya Okada "a boring man", purportedly as a joke.

"She will need to find experienced and cautious advisers as soon as possible," he said. "The difficulty, though, is if she becomes overly careful in opening her mouth. Then her biggest strength - her sharp tongue - will be lost too."