Grimm first met Rei when she asked him to get rid of a shark in a Konoha cafe shop. Not only he didn't do it, he also managed to get more monsters in there for no reason ><
After talking for a couple of months they became friends and soon started to flirt with each other, always in a friendly way, since Grimm was always in love with someone else and Rei was always someone's gf (I know T_T). They fought only once, as a battle training and he probably let her win.
After a year or so, their flirting games became much hardcorish, the epitome of which was the epic day when Grimm flirted (unknowingly and we hope unwillingly) with V.
Also, she was present when his last baby was born, when he tried to take over Higure, when he tried to have msn smex with others, when he fell in love with girls much less beautiful, kind, caring and goodhearted than him.
Their funny moments include the day when Rei changed his blue colored username to sparkling pink

p), the day when she spammed his msn with Spice Girls' songs, the day when Grimm dared her to make a cheesy thread in chatterbox (don't search for that people, it got buried), when he sent her dirty lyrics of dirty songs and so many others...
Currently, they can't talk very often since they are both very busy and have different timezones but they love each other deeply and both of them know that. Only weird thing is, they never had *** with each other, wonder why lol.
love ya Grimmy <3