YMMV / Two Can Keep A Secret

• Alternate Character Interpretation: In-Universe, Ellery wonders if Caroline Kilduff is simply an Aloof Big Sister after watching her be moody about her little sister pretending to be a princess or if girls with fancy dresses and royal titles are a Trauma Button for Caroline after the murder of their Homecoming Queen older sister (Ellery suspects the latter).
• Complete Monster: The "Homecoming Stalker," whose true identity is model citizen and upstanding lawyer Peter Nilsson, is a sexual sadist who relishes his power to seduce teenage girls, plying them with expensive gifts, sleeping with them, and finally killing them once he gets bored and moving on to the next; Nilsson has claimed at least three victims this way, plus a fourth whom he ran over with his car as a thrill-kill. Nilsson emotionally blackmails his daughter into planting fake clues to cover for him, and when heroes Ellery and Malcolm discover his identity Nilsson cheerfully attempts to gas them to death—though not before taunting Ellery with the fact that the mysterious death of her aunt was actually his first kill, and in fact he thought it was Ellery's mother.