Jonadab (M)
First named in 2 Samuel 13:3-35
Jonadab, son of Shimeah, David's brother, is introduced as "a very shrewd man" (2 Samuel 13:3). When his cousin Amnon, David's firstborn, grew lovesick and frustrated over his desire for his half-sister Tamar, Jonadab asked why the king's son looked so haggard morning after morning, then devised the plan: Amnon should feign illness and ask David to send Tamar to prepare food for him alone in his room. Amnon followed the scheme exactly, and used the private access it created to violate Tamar despite her pleading (13:4-14). Jonadab's cleverness here is not neutral -- Scripture records it without softening what it enabled, one of the darkest episodes in David's family. The aftermath consumed the royal household: Tamar's full brother Absalom took her into his house desolate, nursed his hatred of Amnon for two full years, then had him murdered at a sheep-shearing feast (13:20-29). When the news first reached David as a rumor that all his sons were dead, it was Jonadab who spoke up with striking composure: "Do not let my lord suppose they have put to death all the young men, the king's sons, for Amnon alone is dead" -- knowledge so precise it strongly suggests Jonadab already knew of Absalom's plan for revenge (13:32-35). Jonadab disappears from the narrative after this, his shrewdness ultimately serving as an instrument of tragedy rather than wisdom.
References: 2 Samuel 13:3-35