Complete Monster: Rhea Wits, the Mysterious Bust Killer (and Apollo's partner), murdered a dozen people including her brother. The killer tricked the innocent Robert Erlenmeyer into believing he was responsible for the murders, and made him confess. After Erlenmeyer's escape, the killer planned to kill him and hide his body so the police would be left searching for a dead man, leaving the killer free to kill again. After the judge witnessed the killer trying to bury Erlenmeyer alive in a cemetery, the killer decided to just kill the judge and frame Erlenmeyer for his murder as well. After being accused by Apollo of being the true killer, the killer taunted him and implied that Apollo would be the next target. The actual Mysterious Bust Killer, when finally revealed, said that at least they made better use of their life than the victims did of theirs.
Harsher in Hindsight: EVERYTHING about Rhea Wits is harder to think about/watch again the second time, because everything about her, from her Plucky Girl personality through her cutesy appearance to her romantic interest in "Polly" is a big fib. A sham. At her core, Rhea is a Straw NihilistSerial Killer who cares about no one but herself, which makes it painful to go through the game again, knowing she's just stringing you along, ready to stab you in the back the moment she's done with you.
Strangled by the Red String: Subverted. The relationship between Apollo and Rhea seems to progress unnaturally quickly. However, it eventually turns out that it was simply a ploy on Rhea's part to get inside information on the case. Lampshaded when Apollo, after learning that Rhea is the serial killer, curses himself for trusting her so easily when they only met days ago.
Take That, Scrappy!: Most of the Mysterious Bust Killer's victims from the backstory who aren't relevant to the case are OCs with punny names. The exception is Benjamin Woodman, the ventriloquist from Turnabout Big Top, one of the least popular cases of the series.