Who is the cab driver?

Although the ending of “The Demon Lover” is somewhat ambiguous, the driver’s identity is linked to Kathleen Drover’s ex-fiancé. Twenty-five years after he was presumed dead, Kathleen discovers a note that reminds her of a promise that she made to him to meet him on a certain day at an hour that she can no longer remember. In fact, Kathleen realizes that she can remember very little about him and cannot even recall what his face looked like.
As the story progresses, Kathleen becomes increasingly anxious and decides to flee her house and seek safety with a taxi. Her decision is ultimately ironic. When she enters into the taxi, she realizes that the driver starts leaving even before she provides a destination. Moments later, the cab stops, and the story ends with her face-to-face with the driver:

Mrs. Drover’s mouth hung open for some seconds before she could issue her first scream. After that she continued to scream freely and to beat with her gloved hands on the glass all round as the taxi, accelerating without mercy, made off with her into the hinterland of deserted streets.

Scholars disagree about who exactly the cab driver is, and it is certainly left to some interpretation. The title suggests that he might be some demonic form of her ex-lover, and some scholars have argued that he is a wartime hallucination brought on by the stress around her. Regardless of how you choose to interpret his identity, it is important to note that his return is historically symbolic: the lover was presumed dead after fighting in World War I, and he—or at least the idea of him—reappears during World War II. Her ex-lover is less of a person than a historical occurrence, and their time together was not romantic so much as it was transactional and lacking detail. In this way, the cab driver—whether it is her ex-lover or a hallucination—is about the return of the horrors of a world war.