Grand Theft Auto V's writing is largely decent, but ruining Marnie Allen's arc from GTA IV with the Epsilon Program is one of its biggest missteps. Although it doesn't really affect the main plot, it can still leave a negative feeling for those who thought Marnie had made it out okay in the previous game. As GTA 5 reveals, Marnie relocated from Liberty City to Los Santos and fell in with Chris Formage's religious cult, undoing her happy ending in the process.
In the Grand Theft Auto series, stranger missions are lines of missions, usually fairly short, involving a minor side character. They usually have nothing to do with the rest of the story, but offer more things for the player to do and give the protagonist a new person to interact with. Marnie is one of these stranger missions in GTA 4, being introduced as a sex worker with substance abuse problems. Similar to Grand Theft Auto 4's uncontrollable drunk driving, Marnie's story shows the hazards of gullibility and addiction. Although Niko only encounters her twice, her short arc is still quite effective.
Niko's encounters with Marnie do a lot to help humanize him, even with all of the crimes and brutality he deals out over the main story. He feels bad for her, gives her some money while gently turning down her advances, and helps her out again later when she gets herself in trouble. It helps that Marnie herself is kind as well, despite the hardship she's faced. After her short arc is over, she even e-mails Niko to let him know that she's back with her family and going to college. She's a character who brings out the best in Niko, showing his more empathetic and protective side. It also shows that despite Niko's pre-GTA 4 backstory and criminal history, he isn't completely irredeemable. It makes her brief role in the story satisfying.
Grand Theft Auto 5 Needlessly Undoes Marnie's Happy GTA 4 Ending
Unfortunately, Marnie makes a return in Grand Theft Auto 5, having taken a turn for the worse. Despite previously studying to be a psychologist, it's revealed that she became a member of the Epsilon Program, a cult that scorns psychology and other medical practices. Despite the cult's leader Cris Formage being a con artist, Marnie seems to have truly bought into the cult. As an Epsilonist, Marnie introduces Michael into the program, which mostly consists of him paying money, doing busywork, and securing more wealth for Formage. To boot, the questline doesn't even feel satisfying until Michael has the chance to rob Formage at the very end, being a good example of where RDR2's stranger missions outdo GTA's.
The derailment of Marnie's hopeful future in GTA 4 feels unnecessarily unpleasant. If Rockstar was determined to do a storyline about cults, it didn't have to drag in a character that the player already went out of the way to help in a previous game. Instead, it leaves the player feeling like their past efforts didn't matter, because the game's canon determined them to be all for naught. Most of the time, people like to feel a sense of accomplishment after doing something in a game, and their victory being retconned robs them of that. The end result is unnecessary bleakness, especially considering that GTA can already be too edgy at times.
Marnie Allen was never meant to be a major character, but her story deserved a better ending. Maybe she can come back in GTA 6 and get back on the right track, but that seems to be a long shot. As it stands, Marnie's treatment in Grand Theft Auto V was a pointless destruction of a decent character's story.