On the day when Jeremy Corbyn won the leadership for the first time, Len McCluskey was the first person to wrestle the new leader into an awkward bear hug on the small stage in the Westminster pub where his team celebrated his victory.
When Mr Corbyn has hit trouble, Mr McCluskey - the bruisers' bruiser - has been on hand to buttress Mr Corbyn's support, or even to warn off his critics.
But now, enters the contender who will take the Liverpudlian on. Gerard Coyne, the West Midlands secretary of Unite, will announce later on Tuesday that he will fight his boss for the chance of capturing the crown.
Mr Coyne has worked for the union for more than 20 years, having joined as a shop worker at Sainsbury's.
One of his supporters said he felt the union must return to putting its members' interests ahead of party politics. Mr Coyne is understood to be concerned about Mr Corbyn's anti-Trident stance, given that thousands of Unite members work in the defence industry, as well as worried about workers in the auto industry after reports suggested that the Labour leader was considering advocating a ban on petrol cars.
But it won't just be a fight between the two men. Many of Mr Corbyn's supporters will see the challenge as another attempt to undermine the Labour leader.
On the other side, many Labour MPs will see the chance of shifting Mr McCluskey as a way of changing the party's troubled dynamics.
It will ultimately be up to more than a million Unite members to have their say in the spring. But far from being an obscure exercise in ticking boxes on a ballot, it will be the next skirmish, a proxy for the battle over Labour's future.
Few individuals have been as vital to preserving Jeremy Corbyn's position as Len McCluskey, the leader of the country's biggest union.