First of all..the head to head is like 7-2 in Grafs favor...not much of a rivalry really as Graf won pretty much all their matches.
As for the matchup...well its been said. Martina played lime a chessmaster and always tried to look 3 or 4 shots ahead. She tried to blueprint everything out in advance and against Graf there was no blueprint because there was almost no tactical weakpoint to bait her to. Hingis couldn't deal very well with a shot by shot match and reactive play...she wanted to be in total control and when you did not let her she was very beatable as proven by Graf then later Davenport and the Williams sisters.
As for personalities...Hingis was no sweetheart. She had a big mouth and was not all that respectful of her opponents. She ditched Jana as a doubles partner for being to old, then she slammed her one time doubles partner Kournikova in singles when asked about their budding rivalry saying "I win all the matches". She also said of Graf something like "her time is over now" big words seeing as she usually lost to Steffi. Not to mention likening Mauresmo to a man before the 1999 Aussie finals. You may say things about Grafs personality if you like...Hingis was no peach
But to coin Hingis herself here...
What rivalry...Graf won most of the matches...
And yes I know age and all that is a factor...but I could not resist