Raiju was a colonel in the GCPD and Griff's superior. When the war exposed how little the force was doing for people, she took a public stand against it and left to work the streets directly.
She and Griff fought constantly before that. After she walked, she watched him grow in popularity and then lose his partner, and they started working together anyway.
She joined Kato before Griff did, making her one of the League's founders. When the war ended and a newly promoted Griff invited her back into the police, she agreed on one condition: no more colonel. She wanted to be near people. He made her his partner instead.