Freda Clay

In Curtain, the last Hercule Poirot novel, Freda Clay was the niece and companion of invalid Miss Sharples. Miss Sharples died of an overdose of morphia. Freda admitted that she accidentally gave her too much morphia to her in the desperate attempt to ease the elder lady's pain. The police believed that the act was deliberate, but there wasn't enough evidence to prosecute.

Freda never appears in the novel.

Role in the novel; contains spoilers
Miss Sharples was actually the third victim of Stephen Norton, who mentally tricked Freda into killing herself.