Diesel

Diesel appears in the four holiday novellas: Visions of Sugar Plums, Plum Lovin', Plum Lucky, and Plum Spooky. Originally born Swiss, he is described as "over six feet of gorgeous, hard-muscled, slightly tanned male" with brown eyes and thick, unruly sandy blond hair with a dangerous set of dimples. Although not nearly as significant as Morelli and Ranger, Diesel and Stephanie share a mild flirtation. But like Morelli and Ranger, he also has affectionate nicknames for her and is just as every bit amused at the situations she finds herself in. He has special abilities and his job is to track down fugitives with supernatural abilities, called Unmentionables. He pops up every once in a while in Stephanie's life and always sleeps in her bed when he is in town (staying at her apartment and sometimes going over to dinner at her parents'). In Plum Lucky, when Stephanie mentions Diesel to Ranger, Ranger shows exasperation and when asked if he does not like Diesel, simply says, "We don't hang out together." After finding out about Stephanie's involvement with Diesel, Ranger warns her about Diesel's enemies. Stephanie has made it clear that she is not looking for a relationship (already possessing a boyfriend, to whom she is "practically married" Twelve Sharp, and another, even more mysterious man in her life, whom she has admitted she also loves), but it hardly stops Diesel from making moves—sleeping on top of her in bed, somehow stealing her bra (while she is still wearing it), etc.

In Plum Spooky, Stephanie and Diesel babysit Carl the Monkey and get stuck caring for other monkeys they pick up as well. When Ranger finds out that Diesel has been carting the monkeys around, he "looked like he might burst out laughing again." After she is seen with him at Pino's in Plum Spooky, Morelli gets several calls informing him and is also none too happy about Diesel's living arrangements with her when he is around. But like Morelli, Diesel resorted to Pepto-Bismol worrying over Stephanie's safety in "Plum Spooky" (after telling her he was getting an ulcer because of her). He thinks it is amusing that she is so gullible about his paranormal abilities. Despite her ongoing romantic entanglements with two men already, Diesel does not mind trying to get her into bed as well and he constantly flirts with her.

In 2010 Evanovich spun off Diesel into the "Wicked" series, Wicked Appetite (2010) and Wicked Business (2012). Carl the monkey is present, the villain, Gerwulf Grimoire (Wulf), and a new Diesel love interest, Lizzie Tucker, a preternatural cupcake baker.