Not the cartoon Rainbow Brite but my superhero character Rainbow Brite.
A while ago I found a slightly anime wallpaper/desktop image of a cute but rough girl wielding a stylized chainsaw against a hippy-inspired playground wall (bright blue with Beatles-esque cartoon imagery of clashing colors).
She is what I see when I think of Rainbow Brite's girlfriend. She is not the badass, foul-mouthed, ass-kicking machine. She is a sweet but strong girl who loves all the wonderful colors Brian can show her but she also likes hacking into mutant alien robots with a chainsaw.
If it were a television show, I can definitely see her popping up in the middle of a battle against mutant alien robots and cutting them down left and right. Afterwards, the party (or cast, if I am going with the TV metaphor) look at her standing there with a chainsaw, their mouths agape. Then Brian steps over, puts an arm around her, and (either sheepishly or proudly, depending on how things play out with him and the rest prior to her arrival) says, "Guys, this is my girlfriend." Then they kiss.
Later on, Brian describes her:
"I am Rainbow Brite.
She's more like Rainbow Six."