The Heather Beam is a neat "weapon" Heather can obtain by defeating at least 333 enemies. Now there aren't that many enemies in a single play-through, so the kills are cumulative. Once obtained, un-equip any weapons you might have on. Just get into "Ready Position" (hold down R2) and press X. Lasers will fire from her eyes. If you just press X once, a few beams will be fired. If you press X multiple times, multiple rays will shoot out. If you hold down X while facing a monster, the beams will fire directly at the monster.
Now, the Sexy Beam is an upgraded version of the Heather Beam. Sexy Beam can only be used when Heather has her "Princess Heart" Costume on. The same mechanics apply of the Heather Beam apply for the Sexy Beam as well.
Both beams take up a lot of stamina, so use it wisely.