Teacher

One of the three Dreamers - you must remove all three to open the way to the Hollow Knight. Monomon rests in the Teacher's Archives (Fog Canyon). To reach her dream you first beat Uumuu, the jellyfish, which you can only hurt by striking it while it's near the charged Uoma - Quirrel helps by breaking the pillars. Then Dream Nail Monomon to lift her seal. Straight after, Quirrel goes to the Blue Lake for Witness - don't miss him. See Watcher and Beast for the other two Dreamers.