Many Spurs fans have never forgiven the Keane for leaving White Hart Lane for Liverpool, returning to London after his so-called dream move turned into a six-month nightmare.
Redknapp said: "I hate it when I hear people criticise him. He played a massive part in keeping us up last year, it's no coincidence we went on a great run when he arrived.
"I felt the atmosphere around the place changed when he came because of the bubbly character he is. I felt he was vital to us last season. I thought 'if Robbie comes back we're going to be okay.' That's the type of feeling he gave me. I felt he was a key signing because he was a personality as well.
"When he plays he's been great for us. It's difficult for people to understand what it takes to make a football team. It isn't always just about having a team of superstar players - you need certain characters who make a difference to you, in the dressing room and on the pitch.
"All the good teams have those type of people. He talks to players in the dressing room and on the pitch, and gets people lifted around him. He gives you all that and that's so important."