Most footballers who played against Manchester United's iconic captain in his prime knew this. Keane was the driving force behind much of the club's glory years when they dominated in the Premier League and Champions League.
A no nonsense midfielder, who happened to be one of the greats of the game, he could look after himself both on and off the pitch it seems.
Darren Bent, the former Premier League and England striker, worked under Keane at Aston Villa, but was well aware of his character as a player.
In his role as host of Drivetime on talkSPORT, he recounted a story former teammate Kevin Horlock told him about Keane defending himself in an altercation in a chip shop, which was reminiscent of a scene from the movie The Gentleman, in which Colin Farrell takes aim at a group of youths who try to attack him.
Farrell fights off four attackers armed with weapons in a fast-food restaurant before carrying on with order nonchalantly.
Read more on Roy Keane
I played against lunatic Keane, disgusting ex-Millwall man, but Big Dunc is the hardest
Roy Keane hints at comeback but Frank Lampard 'ends his managerial career'
Horlock spent six years in Manchester when he played for City between 1997 and 2003 before linking up with Bent at Ipswich.
"He reckons he saw him in a fish and chip shop and he was eating fish and chips with his hood up. Someone went up to him and pushed him - there were two of them," Bent said as he explained Keane's way of dealing with bullies who tried to intimidate him.
"Apparently Roy Keane just finished them both off, picked up his chips and walked off eating them."
It's not actually clear if this is true and has most likely been embellished over the years to enhance the aura around Keane as a man not to be messed with.
Most read in Football
Jurgen Klopp responds directly to Jose Enrique about exit theory in press conference
Xabi Alonso reveals 'Liverpool dream' in resurfaced clip as Leverkusen make stance clear
Fans love timing of United and Everton's posts as Liverpool supporters react to Klopp news
From Liverpool legends to Premier League rivals: six candidates to replace Jurgen Klopp
In any case, Bent enjoyed a good relationship with Keane when the pair worked together where the Irishman worked as a coach.
It was part of a discussion on football hardmen, with Bent remembering the time he played against Kevin Muscat, known as one of the dirtiest players around.
However, it was not the former Wolves and Milwall player Bent said was the toughest. That award went to Everton hero Duncan Ferguson.