Every day, hundreds of men pay Dannii Harwood to enact their sexual fantasies and boost their self-esteem. Sometimes her clothes come off. Sometimes she dresses up as a nurse or a dominatrix.

If a guy is a regular customer, she likely knows his birthday, the names of his children and his pets — even when to call after a surgical procedure.

He may pay her to help him achieve an orgasm, though she is not a prostitute. He may purchase erotic videos from her, though she is not a porn star.

Ms. Harwood is one of the top earners on OnlyFans, where subscribers — mostly male; straight, gay and beyond — pay models and social media influencers a fee, generally $5 to $20 a month, to view a feed of imagery too racy for Instagram. With that access, subscribers can also direct message and “tip” to get pictures or videos created on demand, according to their sexual tastes.