User blog comment:SayuriDarling/The Dark Prophecy Official Discussion/@comment-5896827-20170426180013/@comment-5896827-20170426234335

Rachel is a mortal with a special gift to see through the Mist in ways even demigods can't. She also had the abilities needed to be an Oracle. Calypso is just normal.

Rachel really just guided them through the tunnels with her vision. Unless they need some super see-through-Mist ability, Calypso won't be much use.