Atlus producer Kazushi Wada reiterates the unlikelihood of the popular female protagonist (FeMC) from Persona 3 Portable appearing in Persona 3 Reload. This decision, explained in a recent interview with PC Gamer, stems from significant development challenges and budget constraints.
FeMC's Exclusion Due to Cost and Time:

Initial consideration was given to including Kotone Shiomi/Minako Arisato, the FeMC, even during the planning stages of the Persona 3 Reload post-launch DLC, Episode Aigis - The Answer. However, the team ultimately deemed the inclusion of the FeMC unfeasible due to the substantial development time and costs involved.

Persona 3 Reload, a full remake of the 2006 JRPG, launched in February. The omission of Kotone/Minako disappointed many fans, but Wada emphasizes the impracticality of adding her, even via DLC. He stated that the development time and costs would be unmanageable, making her inclusion highly improbable.

Wada's previous comments to Famitsu echoed this sentiment, highlighting the significantly greater development time and cost compared to the Episode Aigis DLC, presenting insurmountable hurdles. Despite the FeMC's popularity and fan expectations for her inclusion, either at launch or post-launch, Wada's recent statements firmly suggest it's highly unlikely to ever happen.