Her national profile rocketed a few months ago when a picture of her hitching up her skirt to flee from the tear gas at a demonstration was splashed across the front pages. A story at the time of the recent voter registration exercise, headlined "Ngilu beats up official", also did nothing to diminish the vigorous image she was developing.
On that occasion Mrs Ngilu felt she had evidence that the officer in charge of registering the people in her Kitui Central constituency was tampering with the forms so that her supporters would be disqualified. Accompanied by 20 young men she marched up to confront Mwema Malonza, a supporter of the ruling Kanu party, and grabbed him by the lapels as she issued threats. Only the arrival of plain clothes security officers prevented more humiliation for Mr Malonza.
An incident on 12 July, in which she was this time on the receiving end, had more sinister overtones. She was returning from a rally outside Nairobi with a fellow opposition MP when a truck started to follow them. After several miles her vehicle developed two punctures. When she stopped the truck stopped too and its occupants, carrying sticks and powerful torches, attacked the two MPs, robbed them and left them stranded. The senior policeman in the area where the incident took place described it as a "normal highway robbery", but Mrs Ngilu feels sure the attack was political.
The attack testifies to the difficulty of separating violence from politics in Kenya. This tangled knot is being exposed vividly on the coast around Mombasa. In the township of Likoni, a ferry ride from the city, a mysterious militia has bombed a police station and killed or attempted to drive out non-coastal people. The ruthless General Service Unit is expected to move into the area this weekend, after the expiry of a deadline set by President Daniel arap Moi for the recovery of weapons looted from the station. The GSU has a reputation for indiscriminate violence, and the past two days have seen a vast exodus of the Likoni population. More than half its 100,000 people have already taken the ferry out, and a place once vibrant with life is becoming a ghost town.
The militia's aim was to sow ethnic discord. The message in pamphlets and of its attacks was that people of local origin would be safe, but "up-country" people - the vast army of incomers who have arrived in recent years to cash in on the coastal tourist boom - would have to leave or die. The effect of driving out these probable opposition supporters would be to leave a broadly pro-government voting population, but it is hard to see what benefits the government as a whole is getting from the Likoni violence.
Opposition leaders, including Mrs Ngilu, share a view on this. She says it shows the government is either impotent, or is somehow involved. She spares the president blame only by saying he is no longer in control. "Moi is the chief executive of the state, and should tell us who is killing people." Richard Leakey, a leader of the unregistered opposition Safina Party, told me: "I do not think you can blame the president, but I do not think he is blameless either." Mr Moi is seen as captive in some degree to hardliners in his party, such as vice-president George Saitoti and Nicholas Biwott.
Mr Moi, who has ruled since 1978, won fewer votes in the 1992 poll than the opposition as a whole; he got 1.9 million to their 3.3 million. But he romped home because the opposition was divided, giving him the largest single vote, and he fulfilled the constitutional requirement of gaining 25 per cent of the vote in five of Kenya's eight provinces.
No date has been set for this year's election, but Mrs Ngilu could challenge strongly if the opposition pulls together. Seven opposition parties and other non-political groups have come together in the National Convention Assembly to demand minimal constitutional reforms before the poll, and major democratic safeguards after. They do not want Kenya to follow the example of Zaire, where an all-powerful leader, Mobutu Sese Seko, was ousted simply to make way for another, Laurent Kabila, who can exploit all the mechanisms of dictatorship left behind.
Charity Ngilu says she wants to be a one-term president who, through a coalition government (at present not possible without constitutional reform), will usher in "a process of healing, a process of give and take". The declaration that she does not want a job for life sounds more credible coming from her lips than it would from many of her rivals within the opposition.
The daughter of a minister in the Ebenezer Gospel Church, and now married with three children, Mrs Ngilu founds much of her political ideology on a rugged idealism. In her Kitui constituency she made the building of health clinics and water provision priorities. While she genuinely seems to believe that the NAC is "the only group that is going to save this country", with its platform of irreversible democratic reforms and established safeguards against corruption, she correctly predicted that Kanu would seek to divide opposition MPs connected with the group, a process which has since begun.
Mrs Ngilu will have the strong support of the many women's groups that have mushroomed in Kenya in recent years, though she will be fiercely opposed by the women's groups in the ruling party. The role of women in politics has to be clearly defined, she says; otherwise they will remain "choir singers for male candidates". She is a real threat to Mr Moi, because her hold on the Kamba vote in Eastern Province could prevent him getting 25 per cent of the vote there.
But the opposition is by no means united behind her. On Friday Mrs Ngilu's former colleague in the Democratic Party, the old political heavyweight Mwai Kibaki, declared his candidacy. His is not the only powerful male voice in the opposition that will be hoping to muffle her fierce anger. One fact that neither Mr Kibaki nor any other politician will be able to change, however, is this: in Kenya, far more women turn out to vote than men.