His true form is that of a black horse, while his human form is that of an attractive young man with dark hair and eyes (his pupils are white swirls). His eyes are able to bewitch both men and women alike, and he has relatively strong magical powers. He has the ability to purify water and cleanse toxins.
His name "Kelpie" is actually the name of his species, and not his given name Cain. He and Lydia first meet in Scotland. At the time, a middle-aged woman came to Lydia with her tale of a young man whom she was in love with. Lydia discovers that the man is actually a Kelpie, the younger brother of the Kelpie that wants to marry her. After being convinced of their love, Lydia eventually aids the woman and the younger Kelpie in their relationship.
Despite first apparances, Kelpie (Cain) demonstrates considerable concern for Lydia. He has saved Lydia (and inadventerntly, Edgar and Co. as well) on several occasions, and has gotten hurt on one ocassion in her defense. He regenerates quickly when in water. At least twice, he has tried to protect Lydia by taking her from Edgar's side, as he believes that her being by Edgar is dangerous for her. He is also considerate of Lydia's feelings, rather than blaming her when she broke the spirit of their deal (the deal was tha she would remain in the spirit world with him if he would save Edgar), he told her that he would wait until she no longer had anything holding her back to return to the human world.
He continues to stand by Lydia even after she chose Edgar over him, and he once told Lydia that she is still the first fairy doctor to have obtained a Kelpie. The Kelpie's are feared by most over beings (do to the law of the fairy world), the feed on almost anything they can kill, including faries.
When Edgar first meets Kelpie, he takes an instant dislike to Kelpie, because of his good looks.
Edgar believes that most of his own allure comes from charming looks and silver tongue, he's wary of other men with similar beauty.
Typically Kelpie's will take a female human form. Kelpie's own male form with his dark locks of hair may be a nod to the Manx.