From the Super Mario Wiki, the Mario encyclopedia
At the beginning of 18-Volt's story, 18-Volt, headed home after a long day at school, finds Ralphie, whose games have been given to 13-Amp after winning them over. 18-Volt challenges her to a rap competition, which she accepts. The rap 18 x 13 takes place on Emerald Street with a crowd and 9-Volt as DJ.
After 18-Volt's victory, Ralphie receives his games, and 13-Amp admits that she is impressed. She asks him what school he attends, to which he replies Diamond City Elementary. He then walks off, leaving 13-Amp pondering how it is she lost to a fourth grader.
In the game's credits, Emerald Street is seen in front of an image of 13-Amp and 18-Volt.