Keiko is Yusuke Urameshi's childhood friend and love interest.
The two have grown up to be polar opposites, Keiko being the Straight-A student, and Yusuke, the juvenile delinquent. Despite this she is a loyal friend and goes to great lengths for his sake. From bringing him back to life after his death in the beginning of the series, to lending her support from the sidelines during his supernatural escapades, there really isn't anything Keiko wouldn’t do for Yusuke. Being his love interest, their mutual romance doesn't really come into fully bloom until the very end. Nevertheless, it's made quite clear that they care for one another deeply, throughout the series.