My opinions of him were always based on what I'd seen of the Irishman on TV, in the papers and what I'd read in his two autobiographies.
The image I formed was perhaps the same as yours - of a ferocious, unforgiving and uncompromising loner.
A man who has, by and large, struggled as a manager because he can't empathise with or relate to players. A man who savages anyone he feels has let him down.
A lot of that may well be true, but I got a different perspective by speaking to the former England defender Gary Charles - a friend of mine who has been a friend of Keane's for more than 25 years.
The duo first met as teenagers and shared digs together in Nottingham.
These were heady days for both men. They were becoming regulars in the first team and each looked destined for stardom.
Charles was dubbed "the Brazilian", because of his silky dribbling skills and skin tone.
After one game, legendary manager Brian Clough purred: "When Charles plays a one-two, he goes like a gazelle.
"It's so effortless - at first it looks as if he's not moving, yet he's 40 yards up the field."
There were also lots of nights out, plenty of girls - and some scrapes - along the way.
Both played in the 1991 FA Cup final against Tottenham - when Charles was chopped down by that infamous Paul Gascoigne tackle early in the game - and the defender went on to win three England caps that summer.
Although it looked like each player was on the fast-track to the top, their journeys went in different directions after this.
Keane moved to Manchester United in 1992 and we know the rest - he became the club's on-field emblem and drove them to unprecedented levels of success.
Charles's next destination was, with respect, rather less glamorous - a short move across the east Midlands to Derby County. His form seemed to have gone backwards since 1991.
His career was still pretty stellar - he played for a very successful Aston Villa side, for Benfica in Portugal, and also for his boyhood team West Ham - but it never really reached the heights that had once been expected.
He had also sunk deeper and deeper into alcoholism. Things got really bad during a year out with a career-threatening ankle injury at Villa, and there was an infamous incident towards the end of his career, in 2002, when he collided with another car in London, and told the other driver he was a bank robber on the run before fleeing the scene.
He was sent to prison for four months for drink driving in 2004, then jailed for a year in 2005 for threatening a nightclub bouncer while serving a suspended sentence.
Drink was always at the heart of his problems. I'd need at least another 5,000 words to try and get to the root of why Charles drank - perhaps that's for another day.
This is where Keane comes in again though. The pair had kept in touch sporadically since parting ways at Forest.
When Charles was serving his sentence in Rutland Prison in 2006, he received an unexpected and lengthy letter from his old pal, who was by now the manager of Sunderland.
The letter talked about what good friends they had been and what good times they had had, before going on to say 'There but for the grace of god - what's happened to you could have been me'."
When he was a wealthy Premier League footballer, Charles found he had dozens of friends. Mysteriously, they all disappeared when he experienced problems with drink and the law.
In the letter, Keane vowed to help his old pal, saying he'd give him some guidance about starting a coaching career, and that he'd provide him with somewhere to stay when he got out of prison.
And he was good to his word. In fact Keane didn't just offer his friend a place to stay, he brought him in with him and his family.
The Irishman also took Charles to training at Sunderland, where the former full-back sometimes took part alongside the other players, and at other times organised training drills.
When the side went on a pre-season tour to Holland, Charles went along as well.
The east Londoner has now taken some of his badges and is a coach at the University of Nottingham, as well as being a part-time scout.
It's not all been plain-sailing, but he's stayed sober and is doing well, and it's quite a transformation and an achievement for him. He is also looking after two of his three sons, with the eldest having left home to take up a job.
Charles himself has to take the credit for turning his life around, but a bit of credit should also go to Keane.
Not that he'd want any credit though. In fact he'd hate that. As far as I'm aware, the Irishman has never told anyone about this. He hasn't spoken about it in interviews, or in his books.
The last thing he's wanted is a pat on the back for helping out his mate, in private.
And Charles hasn't wanted to talk about it either, which is why there aren't any quotes from him here, and why it's in a blog.
But I thought it was a story worth telling.