The Rapture Logs/WMG

Fentzy is "The Harlot."

 * Or it might be Donnie!
 * Or The Harlequin.
 * It might be Anna!

Donnie and Anna are parallels.
It's so obvious, especially with the newest log! Both girls hate each other, want Jordan all to themselves, are dominant, and as of the newest log, batshit insane! They've both undergone horrible eldritch torture, too.
 * All the Rapture protagonist girls are dominant. The Harlequin, too.
 * For that matter, both Anna and The Harlequin have a Joker-like smile and a love for wince-inducing painful rape. Maybe they're parallels?
 * With the way the story's going, I wouldn't be surprised if Donnie grows both the above traits as well.