Mercy, as her name suggests, is the quiet and gentle cousin of Kit who has to bear a disability that makes life for her very difficult. However, in spite of this disability, she is the possessor of one of the most amazing pairs of eyes Kit has ever seen in his life. Note the following description of Mercy when Kit first meets her in Chapter Three:

The second girl had risen more slowly, and at first Kit was only aware of the most extraordinary eyes she had ever seen, grey as rain at sea, wide and clear and filled with light. Then, as Mercy stepped forward, one shoulder dipped and jerked back grotesquely, and Kit realised that she leaned on crutches.

Mercy, throughout the novel, is a calm and constant presence compared to her more tempestuous and headstrong sister Judith and her cousin, Kit. She is naturally caring and loving by nature, and this is shown by the way that she does her best to protect Kit from the jibes of her sister and also through her quiet loving from a distance of John Holbrook. She is used as a foil for both Judith and Kit to show the value of submissive silence.