Purse, now of Sheffield Wednesday, played with Johnson at Cardiff City and told the Sunday Mercury: "I spoke to Roger the other day and I think it is a great move for him because it gives himself the opportunity to test himself in the Premier League.
"He rang me up to ask what the club is like, what the fans are like and what is around in the local area, which is something that all footballers want to know. ''Now it's a fresh challenge and a chance to pit himself against the best players in the world.
"Blues fans will love him because he really has that never-say-die attitude. He's got all the attributes you need to be a fans' favourite at Birmingham.
''He will always go out there and give you 110 per cent, he'll play through injuries and fight for every cause so there's no doubt that the Blues fans will be on his side.
"He's good in the air and he doesn't let anything beat him, he absolutely hates losing. He is just a great player to have on your side.
"Another thing about Roger is that he scores goals as well, He's scored between 15 and 20 goals, which is a fantastic record for a centre-half, and he's a big threat in the opposition's penalty box."