Jim,

The mermaid in Child 42/85 has certain powers. She has the power to turn from a "pretty fair maid" into a mermaid. She has the power to bring Covill/Collins into the water where he becomes a fish (he can breathe underwater, etc.). She has the power to cast a spell on him which gives him a headache that causes his death.

Her love for him is represented in Child 42 when she is washing his sark on a marble stone (which has been reduces in the Johnny Collins version to washing a marble stone). She casts the spell on him when he leaves her for his mortal lover after she offers to have him stay with her and become a fish (Child 42C). This spell in not immediate but it can't be undone.

She loves him and when he sees her again she warns him of his death (the spell cast on him). She also brings him into the water and seduces him (and makes him a fish again). Afterwards he swims home to his father's house and dies.

Richie