Angela Eagle is a relative unknown to the public, but her steady if not quite meteoric rise in Labour politics has seen her plan a leadership challenge to topple the party membership's chosen leader, Jeremy Corbyn.
Here's everything you need to know about her.
1. She is the first openly gay female MP in the UK
When she came out in 1997, Eagle was one of the youngest MPs in parliament after she was first elected in 1992 as MP for Wallasey at the age of 31, and is also a former child chess champion.
She embodies modern Labour values as a gay woman with working class roots who now represents a Merseyside constituency.
Eagle has a partner, Maria Exall, a trade union activist, and they had a civil partnership in 2008.
2. She studied at Oxford
Eagle was born in Bridlington, the daughter of a print worker, and went to a comprehensive school in leafy Formby on Merseyside, where she joined the party aged 17.
She studied philosophy, politics and economics (PPE) at St John's College, Oxford, and after graduating worked for unions as a researcher, press officer and parliamentary liaison officer before being selected as the Labour Party candidate for the constituency of Wallasey.
3. She is a champion of women's rights
Eagle is seen as being from the "soft left" of the party and is a champion of women's rights, diversity and the rights of minorities.
4. She has a twin sister
Eagle is older than her twin sister Maria by 15 minutes.
They are the first set of twins to be elected as MPs and later became the first twins serving as ministers of state.
5. She rose to prominence after promotion by Corbyn to frontline politics
While her appearance arguing for Remain in a live EU referendum TV debate was not seen as a resounding success, ironically her greatest exposure has come after promotion by Corbyn to frontline politics as shadow business secretary, tackling the Government with some success on the floor of the House of Commons.
Famously, David Cameron was roundly rebuked for telling her to "calm down, dear" after she challenged the Prime Minister over NHS reforms.
5. She ranks highly among her peers
Eagle consistently ranks highly among members in a list of shadow cabinet rankings, topping the table in the most recent poll compiled by LabourList, a grassroots party members' website.
But she is criticised from the left for some of her past policy decisions - she is strongly pro-Trident and voted in favour of the Iraq War and air strikes in Syria.
7. She is seen as more pro-business than many on the left
Eagle is also perceived as more pro-business than many on the left and criticised for being one of the 184 Labour MPs who did not vote against the second reading of the Conservatives' Welfare Reform and Work Bill in July 2015, proposing cuts to welfare.
However, analysis of her voting record by theyworkforyou.com shows her having voted against the "bedroom tax" and consistently voting against reducing spending on benefits.
8. She served in junior roles in government
In Parliament, Eagle served in junior roles in government, but was sacked in a reshuffle by Tony Blair and spent five years on the backbenches, serving on select committees, with her speciality being economic policy.
She rose again under Gordon Brown and out of government under Ed Miliband.
After Brown's 2010 defeat she came joint fourth in the Labour Party's shadow cabinet election and was appointed to the role of shadow chief secretary to the Treasury by then new party leader Miliband.
9. She has been a minister and shadow minister in four different departments
Eagle has been a minister and shadow minister in four different departments, been the vice-chairwoman of the Parliamentary Labour Party, and chaired party conference, Labour's National Executive Committee and its National Policy Forum.