The Belgian international arrived from Werder Bremen with a reputation for being a top young player in Germany.
But he had not impressed at Chelsea in the Premier League in the early part of his career.
And Kompany admits he took some time to warm to De Bruyne.
"He came into the national team when he was very, very young and at the time he really didn't leave a good impression with me," said Kompany in a recent documentary about De Bruyne's career.
"I thought he just seemed to moan a lot, he tried a lot of things that weren't coming off at that time when he came and I said it's a shame, because we could have done with some more talent in the team.
"Anyway, as time went by and then things happened, all of a sudden, before you knew it, he's the best player we have. I knew it all the time!
"He has an edge, and that's maybe where, in his first session with the national team, way back when he was 18, was maybe the thing that didn't come across so well.
"It was because he has an edge, and then he didn't perform on the pitch, and you're like 'well you have an edge and you don't perform, what's that all about?'
"But later on you realise that's his true strength, you know. He has an edge in everything he does and he's got a bit of a mean streak about him as well, where he doesn't mind getting physical.
"To get the best out of Kevin, you need to play in a certain way. You never get the best out of Kevin if you aren't an attacking-minded team, and I mean in attacking in every way – pressing, possession, tacking.
"So in terms of Kevin coming to City, and Kevin meeting Pep, really, their paths crossing was the perfect blend for him.
"I don't think he would have ever have got the most out of his career, unless he had worked with Pep."